odd engine number

I have just got my 1968 t100t Daytona on the road, it had been in a shed since 84 or 89, I knew it had an engine change in very early 70s, it is stamped up as this in the old green log book. the odd thing is the engine number, SR2400. I have asked a lot of guys about this and speculation is that it may have been a spare engine in the comp shop the bike came from Ken Heams in Hook? England, he was an ISDT rider and a triumph dealer. I don't know if he put this engine in but the bike has always been local. since getting this bike and investigating this number I have been offered a choice of 2 scramblers that have come in from Sweden ( I think) from an avid collector and former rider. these have t100 engines in them and both start with SRxxxx numbers, they are of similar age, does anybody know of this . cheers Neil (aka Mudplugger)