"Were these props (depicted by the Matt Stein Models' "J-propeller" fitted to Skipjack, Geo.Washington and Ethan Allen class from the beginning, or was this applied at refit?"
Craig, I'm not the best expert - exactly why I reposted the question here on ModelFleet - but, as I understand it the "J"-type propeller made its debut appearance in the mid- to late '60s (certainly by the '70s), and were fitted to essentially all boats, as soon as they became available. So, any boats built late enough (possibly the last of the Benjamin Franklin class boomers?) would have launched with the J-prop fitted from the outset.
However all the classic, earliest-launched "name-ships" of those classes - Skipjack, Geo.Washington, Ethan Allen, Thresher/Permit, etc. - would certainly have been refitted with J-props, replacing the earlier, 5- (round) bladed "Yard" props, with which they first went to sea. For such boats, I don't know (even how to look up) the exact dates of their first J-props' installations - you'd have to (first) check all available photos, and then the boat's TROM for the most likely yard period in which the switch was made.
But then, at ModelFleet we also have some REALLY expert (ex-)sub jockeys, who might be able to tell you precisely - or perhaps some better way(s) to find out.
Meantime, Welcome Aboard ModelFleet, Craig!