I made a nice find of rare Bolton material from the 1970's, before he did work in the U.S.
One particularly hard to find, his work on PLANET OF THE APES ANNUAL hardcovers, from 1975-1977, given the fanbase looking for anything apes-related.

Here's a link to the 1977 ANNUAL.

In particular the double-page spread on the inside cover.

And 3 comic book stories:

There are also Bolton illustrations accompanying text stories, but these are the three comics stories. Bolton's work in the other
PLANET OF THE APES 1976 ANNUAL
and
PLANET OF THE APES 1975 ANNUAL are illustrations accompanying text stories, and title pages. But nice early work.
It's surprising it took till 1980 for someone at Marvel to notice Bolton's talent!