My dad has a higgens 30 model # 583.74, without the leftside engager. What I found was Sears owners parts list for model 30 583.70 -583.74. Revised January 1954 so the model 583.70 would fit in the 1949-1950 year. I would guess that.
Problem with mine is it won't auto load, in effect a single shot, been like that as long as I have known. The shell would get caught by the action bar as it came forward, the shell wouldn't lift high enough to slide into the chamber and jam. Trying to find parts.
Also the model 30 is accurate, very accurate. Tack driver even with the 4 power kind of hazy(due to the age) scope.