Definitely not a B-flat trumpet... maybe an E-flat?

Might it be somebody's experiment in altering the spring rate/characteristics of a rubber spring? The narrower rim would make a standard spring deform differently. 

Which one is which? In the photo, one seems to have a wider flange than the other one. Which one is longer? I checked on the MKII Moke subframe I have, and it has the wider flange.

I just went back and looked at several of the original BMC parts books. Although they don't spec out the size of things, I did find several different part numbers for the rear suspension trumpets.

MK1 850 - 2A 7375 welded steel 21A 1031 aluminum

Cooper & S - 21A1302 (it lists VIN from #101 to something which is probably when the hydro cars started)

MKII - 21A 1684 (parts book is labeled "MKII ALL" which to me indicates these trumpets were used on Vans, Estates, and PU, as MKII sedans were all hydro.)

MKI Moke - 21A1032 ("WSE" When Stock is Exhausted, use 21A1684 but add AJH5322 washer at the bottom) This indicates to me that this suspension trumpet is longer than the Sedan trumpet.

MKII Moke - 21A1684 (same as MKII ALL above)

GPO Vans - 21A138 (mail vans and telephone vans)

Sorry it doesn't solve the mystery, but perhaps one of the different numbers are for that particular trumpet. Another mystery is that the MKI Moke uses a spacer under the Front trumpet (which is the same as a Sedan) but not required on the rear with 21A1032. It is used on both the front and rear of the MKII Moke when using 21A1684. Thoughts? Are both trumpets that you are comparing the same length?

It is also a wee bit shorter that the other alloy trumpets I have...not much though.

If a Moke specific trumpet, I have one to go to a new home.

Maybe a Moke they would be light in the rear. Steve (CTR)

A first for me....

Standard rear trumpets, including the early welded steel trumpets for LWB models, appear to have a seat that is 3.7" in diameter.

Came across a rear trumpet with a seat just 3.23" in diameter?

Any ideas??