Johnny: I need to recall some knowlege from my duoprop days. If I recall right, the letter part pertains to the type of drive you have and to the material of the props(s). A "C" would be a stainless prop set for the DP280. A "F" would be a stainless for the DP-S. The number part pertains to the pitch (I believe.)
Looks like you need a "C" propset (if that was the right one in the first place - what kind of a drive do you have?) And if you have the right letter, you need to go down in pitch to go up in RPMs. 3800 sounds pretty revolting though. I'd ask around what performance you should get with that prop. Maybe there are other issues.
Volvo prop letters, decoded:
A: Aluminium, propellers for DP280, 290. Not for KAD42-44.
B: Aluminium propellers for DP280, 290
C: Stainless steel propellers for DP280, 290
D: Aluminium propellers for DP-S drives
E: Stainless steel propellers for DPX drives
F: Stainless steel propellers for DP-S drives.
BTW. I still have a slightly dinged pair of F7's in the basement, nothing that Bossler & Sweezey couldn't fix.