Chip - you got it. its just a board you slide back and forth on. NRS offers one in a stern mount. The advantages to it are:
1. when rowing backwards it makes it easy to get your legs, which are usually a lot stronger than your arms, involved.
2. If you are stacking boats with frames on them the seat doesn't interfere.
3. They are cheap and easy to make.
4. they reasonably fit different sized rowers w/o adjustments.
El Flaco - I would love to see photos of a similar set-up. Thank you.