you can find several articles readily available that describe the history of the Browning BLR rifles but I'll share a little of what I've learned about them that may aid in your search.
they feature a rotating locking lug bolt providing solid accuracy.
you'll also notice the trigger travels with the lever and short throw makes follow up shots a little easier.
the very first TRW BLRs made in late 60's are extremely rare but have popped up on gunbroker in the past.
the first regular production BLRs were made in Belgium and reportedly started in 1971 but I have seen '69 and '70 models. They were originally only offered in two calibers, .308win and .243win.
somewhere around 1973 BLR production moved to Japan. most noticeable difference in the Miroku BLRs is the gloss finish on the stocks as compared to rubbed oil finish on the Belgians.
somewhere in late 70's the .358win caliber was added to the lineup. there are three caliber stamped pre-81 magazines reflecting that later addition.
pro tip on the pre-81 models, make sure you get at least one magazine with the rifle and a spare is a major bonus.
those humped belly magazines can bring as much as $400 as they are very rare with no aftermarket I'm aware of.
in 1981 the BLR-81 was introduced with several changes including a redesigned flat bottom magazine and wider caliber selection.
hope that info helps in your search - good luck!
and as no killer bee post would be complete without some gratuitous pictures so here ya go!
pair of 1971 Belgium BLRs in .308 (top) and .243. I acquired the NOS replacement stock set for the .308 as it has been in the field, the .243 is 98% or better.
some finish variations in early magazines
1984 BLR-81 .308
my 14yo son's first 40yd test group ever firing it
couple pages from 1971 Browning catalog. I find factory catalogs a good place for original specs, features, and photos.
again, hope this info helps in your search - good luck!