Thank you very much everything who replied to my querry about the correct thickness and length of this spring. To those who suggested calling Ron Sitko, I had done that prior to posting this question. Ron had a listing for a "Nifty", but there were a couple of different motors for this machine (I think), so we both thought it worth a try to see if anyone had examined theirs close up.
The motor is indeed a plate-and-pillar style with "NIRONA ORIGINAL" embossed into the upper plate. It may well be that Ron's "Nifty" spring dimensions are good for this barrel, but it didn't hurt to ask. Thanks again for your help. Don