Does look like a special and an H at the start has been seen on some early Malboro frames. The windows in the head lugs is odd/unusual so is the double bottle bosses, These are not typical features with std. Holdsworth frames of the time. That said there is one 'Custom' build in the database that I can see a similar windows in the lugs. You might want to compare yours to this, can you clean up the BB and confirm the number?s
Need help dating a Holdsworth frame.
https://www.flickr.com/photos/28267220@N05/albums/72157624021802289