The mystery shells could have been made by who-knows-whom for a men's store and had a heel pad with the store's logo on it. I've seen that a couple of times - I know Alden has done it for a long time, and I think Hanover may have done so too. I'm sure other makers did.
I can confirm for Hanover - I have a pair of shells that they made for Scott's shoes back in the early 1980s.
And keeping the "grail" brand discussion going, I checked Turnbull & Asser off my list last night, so I think I'm down to Kiton and Luciano Barbera Sartorial from my original list. Wouldn't mind scoring something from each of the Savile Row tailors as well.