Oh man, that is a tough one, I do not really know. I have never specifically collected or studied the M77 models even though I like them and am fortunate to have some pretty rare low numbered guns. There are many different variations and some real rare guns in that lot....Short actions (SA), Long Actions (LA), SA non-prefix s/ns, SA M77"R" model without sights, LA M77 "R" model without sights, SA M77"RS" model with rings & sights, LA M77RS model with rings & sights, SA & LA M77V Varmint, SA with straight contour wood under ejector area, SA without bolt guides, etc.
I know that in production rifles the SA .284 was very hard to find and sought after. Also, all of the Long Action Flatbolts are rare in all calibers. LA Flatbolts with sights or round top receivers would appears to be in the very rare+ category. Several calibers I have never personally witnessed or handled. Only found a 3 or 4 in a lot of years. Had a couple of '06s but never a .25-06 or a 7x57 but was always looking.
Ronnie Burk wrote an excellent book on M77s and it could certainly shed some light on the subject. If you have never read it I certainly recommend it....I do believe I need to read it again too!!
Maybe a more knowledgeable source will come along and enlighten us.