Following a similar thread here:-
https://www.accessnorton.com/NortonCommando/after-market-petcocks.27890/page-2#post-422121
I bought some of the 3 position taps last month for my T140 with Mikunis, and they work well, but the right-angled outlet doesn't work well with Amals on a commando apparently (according to CNW). They're available in the US & Canada from here. Often called Paioli though mine have a different name on them (can't recall what) but may be to Paioli design, they sure look like them (as used on late Triumphs like the TSS and various Italian bikes).
http://www.britcycle.com/Products/621GasTaps.htm
Got totally fed up with the "slosh the trapped petrol over to the other side" shenanigans, and this is on a light-ish Mk1 850 Roadster, I'd hate to do this with a Mk3 Interstate, for example. Taking the tank off seems like a crazy reserve mechanism to me also? So I did the two reserve tap thing on mine only a couple of weeks back.
So confident was I, that the petrol now remaining on the "reserve side" of the tunnel, would take me at least 10 miles (pure guesswork), I cruised serenely past a petrol station last week, confident there was one further up the road. I ran out one mile short, about 7.5 miles!
Ideally I'd like more, and I guess there are variables like how bumpy/twisty the road you're on is, as to how much fuel is left in the "trap", but mine and dommie nator's experiences seem pretty matched?
Wish I'd waited until this thread came up!