Jeff,
Looks like the majority of the boat weight is the sliding seat. The boat must be very light.

I got the reference wrong but here is a unit/ set of plans that looks similar - 19# and could probably be lighter as you said.
http://www.glen-l.com/designs/canu-row/slidingseat.html
The text:
Build your own sliding seat rowing rig. It's easy and fun with Full Size Patterns and simple step-by-step instructions. Built of wood and other readily obtainable materials, the unit weighs only 19 lbs. Designed to be independent of the hull; lift it out as a unit for transporting. Clog spacing and angle are fully adjustable and the seat rolls with nylon sheave wheels on an aluminum rod track. (NOTE: These Plans & Patterns are included with the SCULLING SKIFF.)

Another set of plans that look to be very similar but a simpler crossarm. http://angusrowboats.com/store/index.php?main_page=product_info&cPath=4&products_id=11&zenid=c7b7c1d4de6c4d750f4d6fb2b50571b4

Another set of plans from Ian Oughtred http://jordanboats.co.uk/JB/IainO_Ca.../Snipefish.pdf

Platt Monfort had a tutorial? http://gaboats.com/tutorials/slidingseat_nonav.html

: I built it and it came out at 24 lbs. If I had put lightening holes
: in it I could have shaved AT LEAST 2 lbs and I suspect more. I
: think a 20 lb unit should be no problem. I used oak because I
: had plenty and it is strong. I think Baltic birch plywood would
: work and probably shave off a pound or more. Not that it 3-4 lbs
: is going to do anything but make it easier to handle.

: I have to make some changes, I found the outriggers are just to low
: and I couldn't consistently keep the paddles above the water so
: I have to modify it some. Once I get it dialed in I may make a
: new one just to see what I can get it down to weight wise. Once
: I get it right I will am thinking of creating a set of plans for
: it.