If your lower/gripframe is inch pattern, it should have a strap of metal along the forward edge of the pistol grip and the triggerguard should hook into this. The pistol grip should attach with a screw going up through the forward edge of the pistol grip (under that metal strap) and into the gripframe. The part of the selector lever that you thumb should be rectangular, not round. The inch mag release is wide affair that is the same width as the mag well.
Metric pistol grips slide over a post that protrudes from the bottom of the gripframe and are secured with a nut. The selector lever is rounded. The mag release is very narrow and located toward the right side of the mag well.
You wrote about the magwell being cut with the "rectangular," not halfmoon, slot at the front of the opening and was made by Imbel. This is kind of strange, because Imbels should be metric pattern and the rectangular cut is inch style. If your mag's forward hook is brazed on (inch) and not stamped out of the mag body (metric), you have an inch cut receiver. If the brazed on mags will not fit, you have a metric receiver. Metric mags will fit in inch pattern rifles (but will be a bit sloppy), but the reverse is not true.