Cal. Tissot 28.5-21.
Tissot produced many calibers for Omega prior to the merger.
This caliber was supplied by Tissot to Omega.
According to theses indications, I can tell you that the Tissot watch with movement number 1838649 and reference number 6546-1 was produced on February 17th 1948 and has a calibre number 28.5-21.
I had no idea Tissot used bumper automatic like that, it really is almost identical to the Omegas of the time which makes sense, probably very robust as well