I've had this happen with all sorts of films, but it's less pronounced with Kodak, Ilford, and Fuji films, because they use a high quality backing paper that lets almost no light through the backing paper to imprint the frame number onto the film itself. That IS what's happening with the film you use. The red window works reasonably well with orthochromatic film, which was probably most common when your camera was manufactured. With 'modern' panchromatic film, of course red light will also make an exposure, as witnessed by using a red filter on the lens when shooting.

So, you need to cover the red window up to avoid this from happening. With black gaffers tape and a little piece of opaque photo paper black plastic bag, (the kind that the paper is stored in inside its carton, or substitute other opaque material), cut to size to cover the hole, and attached to the gaffers tape to cover the red window, I also shade the window every time I advance film, and immediately cover it back up again before the camera is let into the sun again.

It's a pain in the neck, but it's just down to taking proper precaution.

(Edit: What Nicholas mentions has happened to me with very old Agfa APX 25 film, and ruined an entire photo trip for me. That was in a Rolleiflex camera that was fine with all other films, but the difference is that these patterns are fairly uniform across the entire film area, whereas the red window problem is only local to where the red window is. Since you use the window to see what frame number you're at, it's natural that the frame number is imprinted on the film, since that is what is always in the red window, except for when you're actually advancing the film to the next frame).