Brian, only disc head code locos can carry ladders, all (see below) disc locos have the four fixing bolts, but it's doubtful all were fitted or remained fitted much after 62, Some of the later built ones may never have had ladders fitted but had the bolts fitted just in case. LMR locos started to loose theirs in mid 61 before the last of the discs coded ones were delivered.
Having said that I've one photo dated 66
I initially dismissed as the photographers fading memory, except midway down the train is a blue grey coach which I think was introduced about that time, 64-66?
What I have just found interesting is that I thought the first ten pilot scheme locos had them and the rest followed, not so, D200-D209 never had nose ladders, bolts or any other fixing, nose ladders only came on the production run from D210 onwards