Update: it fits perfectly, but the LH ball ring ratchet is redesigned on the current hubs and I decided not to trust it with the old-style pawls.
I got a 28-hole 1979 AW from eBay with a nice shell, and put the 1950s AM mechanism in that. Built the wheel last night, fitted it tonight and it works perfectly. With a 13T sprocket and Stelvios the gearing is 62" normal, 54" low and 72" high (compared to 47" low and 83" high with the standard S-RF3 hub). Much more usable, and a little bit more efficient with its oil lubrication and three, not four, planet gears.
I might go to Dunwich on it.
This is the usual chromed steel AW shell, RHS is standard Brompton anti-rotation washer, wheel nut, tensioner, big washer and short Brompton guide nut. Note the 2BA brass washers under the spoke heads.
LHS is an assortment of spacers to get the OLD just right (112mm), Brompton anti-rotation washer and the special AM left-hand nut with a viewing window but no bellmouth guide on the end. The indicator goes right through the axle and cable tension is set with reference to this end. In the photo the hub is in high gear so the indicator protrudes from the axle. In normal gear it's exactly flush, and in low gear it vanishes inside. The reason was to give more thread engagement than the one-piece indicator which screws into the clutch key. It wan't actually necessary - later AM hubs had one-piece indicators and the one on my work bike has been converted by using an AW clutch key.
