MKM Ultimate track frame
Who mentioned 753? I seriously doubt it too, I thought they used mainly 531 DB and sometimes 531 SL in the other tubes except the DT which would make sense due to the higher ST/DT joint around where the tube walls get thinner.
Old Ned, you seem to be knowledgable about Ultimates. Have you ever come across a pure track version before? This appears to have no brazings for road use except brake holes in both crown and bridge.
Will post Frame number when I take delivery. And more pics etc