I bought a keystone k773 camera. What is this knob on the top?

[bdoss2006]

 

[xkaes]

It's film reminder dial for the speed/type of Kodachrome that you have loaded.

 

[Dustin McAmera]

I think it's better than a reminder dial. I think the K773 has an 'electric eye', that is a light cell (I think selenium) which should give you auto-aperture. The knob offsets the meter circuit for the sensitivity of the film. ASA is the same number now usually given as ISO.
I think the camera is for 'standard 8mm' film; that is, 16mm film that's run through the camera once, exposing only half of its width; then turned round and run again, exposing the other half. The film processors split the film along its width, so what you get back is 8mm film with only one set of perforations.

You can still get film in this standard, but only just: Fotoimpex will sell you one type of film, which is inconveniently ISO 100 black-and-white:

FOMA Fomapan ISO 21/100 R 2x8 mm / 10 Meter (Double Normal 8 Film Not Super 8) - fotoimpex.com analogue photography

Cine film Double 8 and 16mm For all 8mm cine-film cameras with film reels (that is, not super-8 cameras). 100 ASA panchromatic film. All Fomapan 100 R films can be ...

www.fotoimpex.com

In the US, Blue Moon keep the same stuff:

Fomapan R 100 Double 8mm Film - Ideal for B&W 8mm Motion Pictures | Blue Moon Camera

Fomapan R100, an ISO 100 double 8mm film perfect for black and white reversal motion pictures. Suitable for 8mm cameras and projectors, comes in 10 meter spools.

bluemooncamera.com

Getting the film processed is another problem. And then you need a standard-8 projector.