About 35 years later it's occurred to me to look up what 'Ulster 71' actually was. I'd assumed it was some kind of innocuous art show, but it was actually the 50th anniversary celebrations of Northern Ireland, unfortunately organised just as it was all kicking off over there. Described by one search result as 'Ulster's ill-timed jubilee'.
It seems these are 3 paintings by Northern Irish artists. Sadly, the Northern Irish government didn't like these stamps when they previewed them, and asked for them to be scrapped and replaced by one with a picture of the Ulster 71 Exhibition, or 'a factory', but it was too late.