I mentioned earlier about wanting the RDB/S handguard to swap out my standard RDB. (If I can get a black one)
I asked KelTec if they would interchange and got this reply:
"As of now, the RDB-S and standard RDB hand guards are not compatible. The RDB barrel assembly is almost 2 inches shorter which puts the attachment point that much further back."
If you count the rail slots from the rear mounting studs to the front handguard pin, there are 21 slots between those points on either model. Also the RDB has a longer barrel than the S ~ (17.3" vs 16.1").
I messaged back these details and asked if they could please double check that. The message was read Tuesday, but I haven't received a reply. I'll update if I hear anything.
I may be completely wrong, but it looks to me like they should interchange. It just makes sense really, plus it measures out. If anyone is ever able to get hands on and confirm this for sure, I'd appreciate it.