Measured on S/A and Plunger lower yokes to hand it's 6 3/4"between leg centres.
From my Parts Books....
'49/53 Parts list shows top yoke as 66 5088 used up to 1953. 65 5430 used next, then another change in 1954 to 65 5448. Later Plunger bikes have a steering lock, I do not have that list or reference, but yoke 65 5448 to hand has the lock.
Bottom yoke listed as 66 5089 for '49-53, and continues to the end of the Plunger Frame.
Dimensionally all variants will be the same with regard to threads, bearings etc. Minor changes eg threads for ancillary brackets, smoothed off lugs etc could be the reason for differing part numbers. I would expect the fork geometry to stay the same, but who knows?
Top yokes generally have a casting number. Lower yokes are all unmarked in my examples. A good bet that any post 1950 A Series lower yoke with the correct form of steering stop will do the job. Drags are worth a look, but sometimes may add to the conundrum.
Swarfy.
Additional. I have a top yoke with forging number 65 5398....looks to have original factory Golden Flash Beige enamel. This does not appear in Drag's list for A,B,M ranges. Casting and forging numbers are usually close to the part number, but not always the case. Plenty of top yokes on ebay this week, some made of gold. Sellers often don't know what's what, but useful as a guide.