Well, the reason for this whole post is because I believe I have found something substantial that will help narrow things down on when the UK mold was made.
It is the crack on the rear right side of the face mask, seen in many UK mold lineage casts. The pictures were posted on several online forums including this one, but if anyone has any issues with me using the pictures, please let me know.
It's on the original face mask that Arnaud owns.
, on Flickr
It's on the GS.
, on Flickr
It's on the TD.
, on Flickr
It's on the TM. (Please note that this picture of the TM is after the paint stripping, but before removing the filler, so some details are obscured - I just don't have other pictures of the TM from that side showing that detail, but TM owners can verify the detail is there - perhaps with a better picture of it.)
, on Flickr
However, this crack wasn't on the helmet originally and I doubt it's there on the Rick Baker mold castings - anyone who owns an SL can perhaps confirm this!?
This is the best picture I can show that shows that side of the face on the original when it was at Don Post and the crack isn't there - sure, it isn't the best angle. This picture is from the Profiles in History auction.
, on Flickr
There are some discrepancies on when Eller let go of the costume and when the helmet should have reportedly been molded for ESB production, but I think it has been shown in the past that the US Rick Baker mold casts and UK mold casts share too many things in common to not be from the same exact source molded at different times in its life - the original screen used helmet - despite there also being differences between the two lineages.
If the conclusion is that the Arnaud face mask is the screen used face mask, and both mold lineages share details with that particular casting, then the UK mold could not have been made at the end of ANH production, nor during the tours, so puts it at around ESB pre-production or even during production, in some way, as both Eller and people at production claims they had it in their possession in '78.
A theory could be that Eller indeed had it until '79, and the helmets used in ESB up until the time production got the original costume back was the ANH spares reworked into ESB, and then additional helmets were made. Though, I hope a paper trail is eventually found to shine a light on this mystery.