Rec appears to offer a reel seat (3 styles) of what look like an exposed blank type, single and double milled with milled inserts. In the discriptions there appears to be option to select a specific i.d. size.
My rod blank has a .341 butt diameter and the reel seat discription has an option for a .343 i.d. Im thinking that the .343 i.d. are (2) sized bushings fitted to the i.d. of the seat with the insert sandwitched in between. If that be the case my guess is that the blank is going to be concentric to the seat i.d. and o.d. and Im wondering if the thumb could contact the blank? If someone has ever used this set-up could you tell me if im on the right track?
Nextly...would one normally pilot the butt grip into the reel seat by reducing the butt grip o.d. to form ' boss dia. that fits the reel tube i.d? What I would really like to end up with is a "exposed blank" spinning set-up.
I suppose the butt grip would not need to pilot in the reel seat i.d but on the casting rod i made the handle kit came with a corkhandle that did fit into the reel seat and it feels very nice, very solid and do folks ever get into piloting the fore grip?
Im resigned for now not to build my cork handle but will use a preformed cork butt grip and could turn the od down to form a boss to fit the reel seat I have but Im still wanting to come up with the exposed blank like Rec offers, sort of back to square one on the handle set up. Im using rec guides so I would like to use the rec seat (the cost makes me want to throw up )but I figure if I do use it I sure want to understand how to set it up correctly. Sorry to get so long winded!

The REC fly reelseats are just bored to the size specified by you, I imagine this reelseat is the same. You can build the boss on your grips if you like, I don't bother as I see no advantage if you have nice tight arbors in the reelseat. The cheap non arborred Wally World stuff uses them because of their sloppy rod construction methods. They sometimes use a sloppy cardboard tube under the reelseat with very little actual adhesive for bonding.