I came to say the same thing. “How many baud was it?” is the immediate question anyone familiar with modems would have, but it is nowhere in the text. Only in the image of the advertisement does it say that it is 300 baud. Because people who would want to buy one would want to know.

My first modem, also around that time was... 10 Dutch guilders (around $4.50 at that time). That's 'cause I bought a suspiciously modem-looking (markings, phone line transformer, line relay, etc.) full-size rack-mount board at a swap meet, no documentation on the thing, bunch of switches and leds on the front, S100-like connector on the back, 'sold for parts'. I traced the circuitry, managed to work out its intricacies, figured out what voltage it needed to work, powered it up and started experimenting. After a few days I had it worked out, created an interface to my C-64 to run the thing through the user port - with pulse auto-dial! - and built ye olde wooden enclosure with built-in power supply and a single-slot rack mount for the thing. It worked for years and was my first way to get online with that machine. It was the classiest modem I ever had, bar none.

I wonder what that board was originally used for, it was quite an elaborate thing, a half-size board sandwiched on top of a full-sized one, a bunch of expensive-looking white ceramic gold-capped circuits on the top board, the bottom one full of discrete components. It must have been something serious and expensive. Still, I suspect the C-64 I ended up connecting it to may have had more computing power than the original host.