Here's a link to the original BF thread about this frame, and other Harding/Holdsworth Pro issues, started by the BF member (shnibop, whose last post was in 2021) who sold it to me:
https://m.bikeforums.net/showthread.php?t=717956&page=1
As to the provenance of this particular frame: one George Harding (from Ireland), who contributed to the above thread briefly, wrote me (via PM) about Hardings after some initial miscommunication. Apparently Harding's shop in Cork contracted with Holdsworth for about 200 frames in the '70s, a pair of which were used by Seán Kelly in Tour of Britain 1976 and by Pat McQuaid to win the Tour of Ireland (with Kelly as teammate) in the same year. He relates the emigration of Charlie Harding, first to Canada, then to LA, where he opened Harding's shop there. As of his writing, Charlie was still living in LA. I couldn't tell you whether the frame in question was purchased in Ireland and made its way to the West Coast, or had been sold by Charlie Harding from LA; shnibop says in the thread above that he bought it on "local CL", which I assume was in the Seattle area. So it's come back home, in a fashion.