Hi Warren-

The plastic butt plates become dominant on Standard and Featherweight M70s relatively early in 1958 (receiver polishing room date), around S/N 435,000 to 450,000. This is considerably earlier than the change from hand checkering to the narrow panel machine cut checkering (1960-61), so M70s with plastic butt plates and hand checkering are common, especially in 1958-59.

I the case of some specialized stocks, e.g. Varmint rifle, that could only be used for one thing, the changes were somewhat slower and earlier style stocks, e.g. hand checkered, can be found much later in production than the “expected” changeover. 

Looking at the survey, the change in 300 H&H MAGNUM butt plates seems to have occurred in the same range as regular standard rifles, despite the H&H stocks being “specialized” (longer magazine well).  OTOH… The changeover from hand cut to machine cut checkering on 300 H&H Magnum stocks never seems to have fully taken hold, as there are quite a few late (1962-63) 300 H&H Magnums with plastic butt plates and hand cut checkering… 

Hope this helps,

Lou