Tim,
Unfortunately, the RNLI archives in Poole no longer (or at least not for the time being, anyway) have their collection of available original lifeboat plans listed online, like they used to; however, if you cannot find the information you are looking for elsewhere, it might be worth your while to get in touch with them: **LINK**. When I looked a while back at least, they certainly offered to sell copies.
It might be that for the moment at least they require one to come to Poole; this certainly used to be the case if one wanted to use their collection of photos. I've no idea how close or or far you are from Poole, but should a visit be necessary, I dare say it might be turned into the occasion for a nice day out … 
In any case, you could perhaps try giving them a ring or sending them an e-mail to find out what's what?!
Good luck and good hunting 
Mattias