https://www.mtbr.com/threads/help-mystery-mountain-bike-frame.676415/
https://secondspincycles.com/1991-doug-bradbury-manitou-april-1991-mountain-bike-action-test-bike/
See the second pic in the second link. Similar but not quite there
They've gone to the trouble of incorporating that wishbone and then used bird poo welding, weird.