John,
The early Marks (Mk1, Mk3 and Mk3 Sports) had frames with a plate lug on the back of the vertical tube, just above and behind the swinging arm pivot, for the attachment of the rear subframe. Later frames (Mk4, Mk5 and 'Plus') had the subframe attached to a boss on the cast rear bottom lug. Quite a lot of the early frames suffered from tube breakages and were replaced under warrantee - some of these replacement frames had the lowered mounting point, and therefore, a modified subframe was also supplied.
From the way you describe your original frame, it appears that it is not a Mk4, but either a Mk1 or Mk3 frame. Mk1 frames were numbered up to 2999. Mk3 frames were numbered from 4000 to 6999. Mk4 & 5 frames - with the later style subframe -were numbered from 7000 to 12550.
Hope this helps,
Regards,
Eddie.