Getting back to the OP's question, as far as I'm aware, haulage bashing took off much later than spotting (probably in the 1970s, around the time the Westerns were being withdrawn).
Well that is just plain wrong. Sorry but I can't word it less bluntly than that.
There were haulage bashers doing D800s before D1000s; at least one is still around today, I can remember him doing it while D800s ruled Exeter Waterloo.
Incidentally, IIRC same person had 510 duffs too. By the same token, I know of two quite seperate individuals who both had 97 D6500s with each one missing a different one. So that put them as early 1960s bashers too. One of those used to have a tee shirt bespoke printed "I pushed Huskisson".
But well before that there were steam bashers long before diesels bashers.
Well known current loco bashers such as The Leader (and if you don't know who I mean then you don't know haulage bashing) started out mid 1960s steam on the Waterloo Bournemouth line, and "apprenticed" to several long standing steam bashers (one of which at least is still around now). And I bet those elders followed on from earlier persons.
I can also point to SR EMU bashers doing things like Cors (so 1972 and before), and before that Bils and Hals (1970/71 and before) and then before Puls and Pans (1964/65 and before).
Back in 1982 when I was doing 25s most summer Saturdays on the Cambrian, it happened the following day was a run with Sarah Siddons, the first for a number of years. I mentioned I was doing that next day, to which one of the regular Cambrian 25s followers said they had had something like three quarters of those LT ex-Met. locos, and the following weekend produced the moves books.
Those are just the individuals I know or came across - there must be 00's of others out there all knowing further such bashers.
Nothing is new under the sun. WW2 may well have introduced a gap in note and number taking, record making, and travel though, and may explain the post war burst in number crunching and travel.
Going way back, many early railways did not number locomotives only named them, and or other systems re-used low numbers over and over (LNWR for example) - E.S.Cox refers to a curiosity then interest in LNWR spotting and numbering - and that predates WW1. ISTR he comments that with names only or with recycle numbers filling in low digits gaps you can't determine how many there are of a type etc, which sparked curiosity. O.S.Nock seems to have had similar beginnings. By then railways were already decades old - and I'd agree there were trainspotter(s) meaning interested amateur observer(s) as opposed to industry interests at the Rainhill trials.
I think the hobby goes back to the dawn of railways - Dvořák, great Czech composer (1841 -1904) was famously an enthusiast and could often be seen at Prague's main station observing operations.
Antonín Dvořák representing composers and contemporary Claude Monet (1840 - 1926) representing artists.