It sounds like the old gent had a number of DCRA conversions, which was very common among the more serious target shooters in the 60s and 70s. The numbers you saw would be the conversion serial numbers if they are stamped on the top face of the bolt handle, barrel ring and barrel reinforce. These have been discussed here and elsewhere online. At least one person is compling a list of serial numbers to see what can be learned from it. (Not much other than highest number seen as the rifles were whatever the owner sent in)