Later leg mudguard mounting consists of a Y bracket supported by two studded bosses welded to the leg. Studs are at approx 25mm/1" centres to match the bracket, top stud centre positioned 78mm/3" from bottom of the oilseal holder thread. Bottom stud centre positioned 159mm/6 1/4" from spindle centre. Studs were 5/16" cycle thread/ (8mm equivalent diameter.)
A word of caution. Welding anything on the legs will distort the bore, and stands a good chance of wrecking them. Better to source the later parts, but in the meantime stick to the original plan to use your early legs and fabricate a bracket to support the later mudguard from the existing mountings. In a word, don't modify genuine original parts, but by all means cut and shut modern repro parts from who knows where.
Swarfy.