ROF Fazakerley Mk 2 serial numbers were pretty munch consecutive up to around April 1953. After 4/53 is when the allocated batch serial number contracts start to appear as you have noticed. I am currently working on a list of observed s/n's for my No. 4 book, what month are your 187K and 373K rifles as I have a large data gap between 3/53 and 4/54?