Paul,
I think you are right re a long hunt.  I just scoured my ebay archive of #10 Griswolds for the last month.  There were no ribbed handle #10s at all in that time.  Which other rib handle Griswold skillets do you have and which are you looking for?  I'll keep an eye out just in case.
The #10 you are looking for should have a heat ring, ribbed handle, and large block logo.  I've seen the heat ring and ribbed handle on slant logo #10's but not on a block logo skillet.  I've seen at least 2 different handle variants on #10 block logo skillets with heat rings.  If there is a ribbed version, that would make 3 variations.  The two variants I've seen were: a flattened place similar to the large logo smooth bottom, also a version with a small rib that extends about 1/4 inch from the side of the pan, then goes to a flat place.
I'm looking for a #10 smooth bottom large logo to complete a set from #3 through #10.  I could have bought a couple the last few weeks but I'm not about to pay the "OUCH" price of $70 that they sold for.
Good luck with the search!