1990 frames have a 'triangle' here:
I thought that, unlike any other frames from the range, Team 1990 and Team 1991 had two cables on the top tube:
1990
1991
https://www.retrobike.co.uk/threads/marin-frs-titanium.491778/
Presumably there are different size ‘rear ends’?
I have a 1993 Team FRS but it’s not for sale.
I have the same XTR 27.0 seat post, which I might try to restore.
There's some suggestion online that heating the metal and agent improves the process.