I recall Chris Pratt (El Primo Commandante of HMCC Sentinel) telling me that the US boats are essentially the same hull and wheelhouse, built in the US under licence from Vospers, but without the huge Ops Room behind the con. There's some very hush-hush, secret squirrel stuff in there. I've been inside it but I'd have to kill you if I told you what I saw......... They also use what he called proper engines - "not b**** Paxman donkey-pump engines".
The rear part of the S/S is longer and lower, and the RIB sits at the back of it - behind a lattice mast thingy. Chris reckoned it was a prettier boat than Nellie; I'm not so sure I agree, but I much prefer the USCG colours to RN grey.
If you want to build a US Island Class model at 1/40 scale (approx 1M long) then Model Slipway might be disposed to supply a hull only. Best contact them and be very nice to Jackie!
BTW Peter; your model of Nellie looks superb on that water.
FLJ