It's an Air American Unireg 320...... way old, but still works a treat, and will do 4500psi. Apparently in the early days of Nitro/Air, some stupid EU law meant that you had to have an on/off valve inline with the bottle, so the UK version uses a screw in tank and valve.
I tried Air America UK (i.e John and Jackie Sosta!) and they don't do a 4500 psi capable valve..... just wondering if anyone else does one- seems a shame to have a 4500psi capable reg and tank, but be held back by a silly tank valve!