According to my original parts book the depth adjusting collar is "Not suitable for heavy duty models, i.e. SILVER and CENTURY PLUS long shaft fixed and clutch drive"
Isn't the book referring to the Century PLUS and the Silver Century PLUS? Both have a heavier drive tube (thicker) than used on the normal centuries and Silver Centuries. The adjusting collar will not fit the pluses but I think they fit the (silver) centuries as good as the 40s.(?)
So thats a Silver Century (non plus)
A depth adjusting collar should fit that then. They fit a driveshaft tube diameter of 1 1/8" Just checked. I have one that I had on my 40+ at one time.
I have the Seagull collar on a 40 Plus and it is great for that. Not suitable for anything heavier in my opinion however and that is actually stated in the original "Makers Methods" leaflet. The collar is just two halves of a nylon type bush with through bolts and I don't think it would survive for long holding up the weight of a Long Shaft Century. Better to through-bolt a block of hardwood to the transom to get the right height.
A boat is just a boat - drill away! The Seagull is a living pulsating creature which needs to be nurtured, fed and well perched.
I have seen two G claps and a block used to perch a Forty Plus on but again I think the weight of a biggy deserves getting the drill out. The splash and squawk of a Gull liberating herself from the transom would be spectacular but short lived.
Robin