G'day Bill. Just put my back out climbing a ladder to take these pics. Getting old, says the 51yo.
The bit welded between the down tubes is 5/8 square and 2" down from the V. The lug is 3/8 thick and 7/8 long, 5/16 hole 1/2" out from base.
Cheers.
Will have to fix that frame one day.