18, 19
32, 34, 36, 38, 39
49
51, 54
61 – 65, 69
72, 75, 78, 79
93-96
Taking those up to and including 51, off the top of my head, without looking back through my BR Diagram Book of 1966, that i can't put my hands on right now, and looking what was in traffic 1966/1967 which would have been when the 1968 series was worked out (because these things do not appear overnight, they'll spend months in committee meetings being agreed).
*Assuming* - without proof - that certain main classes had several class numbers instead of all sub-types were re-merged back to main classes like 72 -> 73 and 34 -> 33, you could get this :
18 = 2nd 17 subclass i.e. what was later 17/1
19 = 3rd 17s i.e. 17/2
32 = 1365 hp Mirrlees BT2 ***
36 = 1600 hp Mirrlees BT2 ***
38 = 10001 <--- this was in traffic in 1966, it is in my Diagram Book
39 = D600 **
49 = 2750 hp BT4
51 = DP2
** I am aware everyone seems to quots 41 as D600; however this was NEVER published in any contemporary BR document. Unless someone here has one - a BR one, not an unoffical publication, and is willing to show it to me, I will always doubt this. There is no reference to 41 as D600s until about 1980 when it appeared from CJMs works. Rather I suspect 41 was allocated to D800-802 and then they morphed that subclass to the main 42s and left 41 free. Further, in the BR Diagram Book diagram DH/400x (I forget the x) for D600 appears in the sheets BEFORE diagram DE/400x (likewise I've forgetton the x) thats is Class 40, hence putting D600 before D200 fits, releasing 41 to be in exactly the right place where D800 diagram preceded D803 etc. HOWEVER this does make one anomaly I cannot fit - D830. - because if D800-802 had their own class then D830 would have, but don't forget I am making assumptions : 41 might have been D830!
*** Mirrlees BT2 as built had 4 power settings 1250 1365 1600 2000 hp. The 2000 hp one had long been downrated, and in time most if not all those that were left were all down to 1250 hp (class 30), nonetheless there were 1365s and 1600s around to about 1965/1966.
((If 1160/1250 hp is enough to split 24/25 and 26/27 then its enough to split BT2 into 1250/1365/1600.))
49 - all duffs were built new with 3750 hp engines except the 5 with the V12 2610 hp engines that became 48s. All the 2750s were rebuilt and downrated to 2580 hp (that became 47) BUT there was still an intention mid 1960s to restore some to 2750 hp. BR bought 50 new Sulzer blocks to do this, they never did it, the blocks ended up in 45s when they made 45/1s.
51 - this is a pure guess. BR had at that time two makers prototypes D0280 and DP2. They bought D0280 as D1200 class 53, there is no reason why they could not have planned to take on DP2, it just got smashed up before they did it. Yes its a guess, but is a good gap filler.
I suggest people who try to do things like 2x are type 2, 4x are type 4 are looking too deeply at this. Think how things are done in the office today. You have a pile of drawings in numerical order in a Diagram Book. You give the lowliest person in the drawing office the task of numbering them into a 2 digit series. They will start at 01 and keep going. They will not come up with esoteric solutions.
Neither did they leave 38 48 blank for new types. This is chickens and eggs. But I suggest the proposed new types got those numbers because they were vacated, not the other way round.
--
Nick
. It was called Falcon iirc and numbered D1200 it had the same Maybach MD655 engines as a Western. It was aprototype that spent most of its time running round in South Wales, freight mainly iirc. It never carried the number 53001 though so I guess that whole class would have always been re-usable. Any rail vehicle number can be re-used if the original vehicle stopped carrying it a long time ago. I don't know what a long time is 
, I think tens of years. Someone else will know.
