The only thing I can add is ...
I have an old custom Venom straight blade on my target stocked 77. It is also set back a tad. I love it.
I'm also wanting to improve the trigger blade on my other 77 which is in a home made thumbhole stock.
I was going to get an extra set back curved one from either Rowan, or being a skinflint, I was going to chuck 13 quid at one of the steel ones from Robb and have a play with that.
I just keep thinking I should stick with the straight option. Using my 77 with the straight balde there is that bit of flexibility re vertical hold. Using the very curved extra set back ones I keep thinking that they look nice but my finger tip will be guided into the 'concave' blade, and will be sitting at the furthest point back ... with no flexibility for up or down on the blade ( maybe that's a good thing as the finger will be at the same point each time ? ). If I want to go down a little then the curve of the blade will push the finger further away.
So I'm probably going to do what Paul did and cut the original blade and make up a straight brass piece to add on ( sorry again Paul ... I know you said you don't like brass