Clem,
Personally I thought the supercharger on the Mark XIV Griffin engined Spitfire bore more similarity. The Rolls-Royce Merlin Mk. III tended to resemble the upper radula in the Late Triassic nautiloid Cenoceras. IMHO, that is.
Anyway, I can't really tell you what is on that early Victorian engraving as it has not turned up in the post yet. I took that photo directly off the site I bought it off so I can't really comment on what exactly is represented just yet. I'll let you know when it turns up.
Phil
BTW The Thomas More painting came into my possession in a similar manner to the large ammonite recently. Plenty of alcohol and a pub was involved!