Scoured the internet ?
did this include going to Oregon, Prime Line. Stens & Rotary catalogues and checking the "Blades by length" charts ?
Removing your blades and measuring the mounting holes I am assuming that you still have the original 1974 2 hole & mounting plate attachment
The only time I fitted these the blade had a center hole + two small holes & they were from a JD
And blade lengths are measured like TV's corner to corner diagionally
It will help greatly if you posted the part number of the blades.
And that model number