Yes it's an older mylar main I believe. Not cheap but much much stiffer (and less forgiving) then the Pentex.
There would be more around except for the price and the fact that most get destroyed long before their time by crew falling into the mainsail during capsize. The pentex are cheaper but even less durable especially with the size and shape of the window these days.
Whose boat is 403? How do you like the sail? I've been talking to Glenn about using the same cloth on a mozzie main.