Thanks, Sue and m. I didn't attach much significance to the initials SH because I thought at best that would be a tribute and hence not helpful to identification of the maker beyond what is obvious from the appearance. I agree they do not appear to correspond to Sam Herman's usual signing of either the S or the H.
I agree they could well be SM, and then Stan Melis is an intriguing possibility. Melis's signature on a few examples to hand shows some variability in both the S and the M, and does not rule out a match.
Another piece I've seen with the inscription JFWI from 1980 has an additional marking that seems to say U2. I'm not aware that Bono and the lads were in Adelaide at that time! On the other hand, it may suggest the letters SH or SM are not somebody's initials at all.
The obvious solution, of course, would be to ask Stan Melis. He has been charming and helpful on the couple of occasion we have met him, but he tends to move around. I'll see what I can do to track him down through other contacts.