How common are these now a days ??

Relatively rare? The early Columbus tubed Treks are definitely up there in terms of value. You could probably turn a pretty penny if you listed that frame on eBay. It looks to be in great shape and it's a popular frame size. The Arabesque 600 group is nice too, though not as value enhancing as a Campy group. Too bad about the rust.

Can't decipher the serial number?

I can't find a serial # on it anywhere..

You may already know this, but here's the authoritative online guide for vintage treks: https://www.vintage-trek.com/SerialNumbers.htm

I've provided the link to the page on Serial Numbers, and how to locate/interpret them. Great information on the rest of the website as well, including brochures. Your frameset is likely 1981 or earlier (if those decals are original), and the Columbus tubed frames were the top of the line.

It looks almost like my '79 trek did. My head badge was held on with 2 little screws and the decals were on the downtube if I remember.

So... '80 maybe? I know they switched to the glued on head badge sometime after mine was made.

Serial number is on the bottom bracket. You may need to sand the paint off to fully see it. I have a 1979 and until I stripped the frame I couldn't see all the numbers. Roger