If I had this frame, I would look again at those frames that most resembled it. Rather than a tiny manufacturer that nobody anywhere can recall, it's much more likely that a relatively sophisticated frame like yours is a pre-production model, or a sponsored rider version that differs significantly from the commercially sold model. That's why you can't find that exact frame elsewhere in order to identify it.
I think you've already seen a picture of the descendant or tweaked version of this frame, but you didn't realise the connection, you only noticed the differences.
The frame having no frame number is another pointer to it being most likely a custom-built for a pro rider or being a development model, not a production bike. Some machines in these categories have no serial no. at all, some have a low number, or just initials, or a combination of both, often stamped in an unusual place on the frame.
Someone who was around the racing circuit when this frame was current might remember it - the marks of extensive use seem to indicate that it might have been raced at some time.