Going back to the seat tube mod...
The extended seat tube was prone to cracking, due to the thin gauge lightweight seat posts commonly use back then, the seat posts flexed and in turn fatigued the seat tube extension.
Duncan's answer to this failure was to cut the extension off and replace it with the RC200 seat clamp, hence your frame. It's hard to tell from the photos but if the black finish is almost a flat black it could be a factory finish after the seat clamp work.
There was a batch of F1 frames that were finished in a black anodized finish by a alloy window firm, giving a super durable black finish, guess yours could have been done at the same time.