Hi Chris, this is a very attractive weight you own!
A couple of weeks ago I bought a Strathearn P7 with uncommon flat polished base, and aventurine core spokes, which is too not a common feature with this model. Only the clear ground P10 signed with S and two digit year from 1969 on had always a flat polished base/as Alan stated. But why?
I thought that Strathearn tried to enhance their products as a reaction to the Perthshire spinoff in 1969, by improvement in some details; Perthshire started with the limited editions of PP11 and PP12 in 1969 and until 1974/1975 these models have exactly the polished flat bottoms; I even have a Perthshire PP1 with polished flat base, which may be from the transitional year 1969-the early PP1 had a fire polished base normally...
I think if it was ground for repair reasons, the profile would have suffered noticeably.