Dyno steel frames, all I ever messed with, were 1" threaded.....
My Glide Deluxe is 1" threaded....
The Alum. frame Dyno's were 1 1/8th".
I say, have the customer measure the stem if possible, or hold a ruler up to it and snap a picture so you can compare.
Some of the DYNO collectors should know this one for sure!