Cheap, made in Spain entry level rifles.  Some folks love theirs, others curse them.  It probably is .45, as you say.  Clean it up, start with a load of about 1 1/2 times the caliber for round ball, so around 67 grains of 3Fg, then work up and down from there to get best accuracy.  I think I would tie it to a tire, then run a long string from the trigger to proof it.  
Nice find, by the way.  You can't complain about a freebie like that!    

-WH-