The 45 and 46 bases may be right some or most of the time. I have had two (one at the moment) that have a screw spacing on the bridge that is larger as on some M70s or .860". Talley 2 pc and Warne (M902/831M) make bases to mate with these receivers. Both Talley and Warne Maxima are steel and heavier but fit. My smith adapted some Weavers for the earlier M50 I had while a couple others used standard FN mounts. Why the difference, I don't know but that has been my experience.
I think the change was because Sears was selling proprietary bases and rings hoping to eek out some extra $$$. I've had them with both types of hole spacing. Sadly, it is all too common to find the Model 50's with a third hole drilled in the rear bridge so that standard FN bases can be used.
FYI, Talley also makes Model 50 specific bases.
