Later type alloy barrel, (HS/HT not VHA/VCH). Head looks like it might have the '56-on push-in exhaust pipe but the timing cover is the 1955 and earlier style with the 'rounded' ear for the top/rear screw for the mag chain cover.
No lights and competition mag?
Also some funny stuff going on with the (incorrect) brake pedal - and I wouldn't trust that gear pedal to stay on very long!
Can't see any blueing on the exhaust, it's very clean and there's a few things to do before it'll run - makes me wonder if this has ever run as a single?
If it's a recent project the previous owner might still have some of the FH parts (it would need mag/dyno/engine plates/chaincase inner and various bolts and studs, plus the engine shaft shock absorber, as well as the engine cases - A10 cases don't fit I'm told...)
Dave - any gearbox numbers or perhaps the engine number that's on the drive-side case between the front engine plates?
(That's a very interesting bike you've got there as well as very shiny!)
'55 Huntmaster, '56 VH, ' 51 VH, '80 R100RT, '00 Sprint ST (but all those Ariel parts can only make one running bike...)