There was a logic, only it varied between types of class. Initially it was being done on works overhauls (or when new). From a starting date in late September-early October 1973 the BRB initiated a full-scale renumbering project organised via the regional CM&EE offices.
I can't speak for other regions, but on the LMR the exceptions were the WCML electric locos, which were done on Crewe works - I may have known why at the time, but forget today. I suspect it was because of the metal cabside numbers - they didn't want those being ripped off at sheds.
It depens where you were, perhaps, but I can assure you that there were plenty of LMR locos renumbered by New Year 1974. We had mostly finished things off by February 74 on the LMR.
AS I mentioned above, the whole operation got going late Sept-early October 73. I know, because I was in charge of it on the LMR! It was my first 'real job' opon graduation. I don't know for sure about the other regions, but as it was a BRB order, they can't have been far behind, and some may have been ahead.
By mid-October, I was visiting depots instructing teams on what was needed to be done.
I can confirm this - at least as far as the renumbering exercise went. It was most precisely specified - although there might have been some local oddities, eg with the in-cab numbering, especially if a depot ran out of the little numerals. ISTR one or two WR 47/4s by 1975 had the numbers painted on in the cabs.
Funny story. As mentioned, I had to go out to all the LMR depots to instruct the local lads on what needed to be done. (Usually one trip was enough to each depot, but I did three teams at Toton, one for each shift.)
Frankly, it felt a bit embarassing, as there was me, this 21-year old just out of uni supposedly telling 40-50-60 year olds how to paint over the old numbers, clean up the side of a loco, stick on the numerals, paint over the lot and then remove the paper covering on each numeral. I mean, it wasn't actually very high tech!
So I did my best to not appear to be a blerk, and explained that I was very sure they could do it themselves without me, but that I'd been instructed to come out to each depot for this. I also added that there were, in fact, there were a number of 'simple' things that they had to get right. For example, the vast majority of the numerals were 6" in height, but those for 08s were only 5", because the shunters did not have enough space for the 6" ones.
Since I had allocated the numerals to each depot based on their loco allocations, it was important not to use 5" numerals on anything other than 08s, or they would find themselves short (and some big shot might notice the smaller numbers on other locos and everyone would get a rocket.)
But the biggest pitfall I nearly learned the hard way.
I guess it was early-November or so when I went to Longsight. Of course, I'd arranged it with the depot manager that I'd be coming, and he lined up a team of two for the session. He had a Class 25 stopped on a D or E exam, so that was selected as the one to do. (We didn't want a loco stopped just for renumbering, of course.)
Now the BRB had sent out booklets with all the loco numbers in columns - old number on the left, new number on the right. We had reproduced these at CMEE HQ, but only half size, IIRC.
Whatever, the shift foreman (I suppose it was) would tell the teams - renumber (say) 5247 today lads - and the team had to look up 5247 and look across to the right for the new number. Let's say it was to become 25 197.
Of course, on this instructional run, I did this - showing the team what I was doing, of course - and the way it went was I renumbered one side, and then the team did the other side. (Only one number was applied to each side, not under both cabs - I suppose to save costs.)
When we did this, a most important step was to report the job done to the shift foreman, who in turn then had to inform control of the change in number.
Of course, with the second side done, I explained this to the team, and we went into the foreman's office where I would tell him 'job done - 52xx is now 25 yyy'.
At this point, my guardian angel, or perhaps it was the spirit of Sir William Stanier, whispered into my lug 'ol: "Before you inform control, you should just check you've got it right."
I say my guardian angel, because I'm sure I never did this on earlier depot trips. I mean, it's not exactly difficult to read one number and go across to the next column, is it?
Ha! So you'd think. The trouble was that on some pages the columns had got somewhat out of register - something I'd not noticed before. It was thus VERY easy to scan across and get the wrong new number - out by a single digit.
And one of those pages was the Cl 25s.
Sure enough, 5247 was not to become 25 197, but - if you looked carefully - 25 196. *
Oh no. There was I, the instructor for all this, putting the wrong number on the loco. What to do?
For a nano second, it flashed across my mind that maybe I could slink out of the office and renumber the loco again myself. But at nano-second plus 1, I realised I was bound to be noticed, and by next morning half the region would know this smart-arsed blerk from Derby had messed up - ha ha ha!
So I just fessed up there and then, saying something like: "Well, would you believe it? This fellah come all the way from Derby to show how to do it and gets it wrong! Look at these columns - it's easily done - we need to re-do it."
And with that, nobody said a thing. But of course, I had to send a note out to all the depots to warn them of this easily made slip.
As far as I know, no loco went out with the incorrect number on the LMR, but it could easily have been different - and we might have had two locos with the same number out on the tracks at the same time - although presumably control would have noticed that immediately the second loco was reported as renumbered - but who knows?
Whatever, sometime in late February - early March my boss came to me to say the BRB bod responsible in London had sent congratulations that the job was complete and seemed well done.
* These are not the real numbers. I don't remember which locomotive was the actual one involved.
