Main differences between the FAL Metric and Inch pattern receivers.
The main difference between the inch and metric receivers is the cut in the forward face of the magazine well. (Picture #1)
The inch receiver (left) has a large machined slot. The metric receiver (right) has a smaller thumbnail shaped cut.
The inch receiver (top) (picture #2) has it's cocking slide track cut off (approximately. .300) to allow the cocking handle to fold down.
The track on the metric receiver extends to the forward end of the receiver. The cocking handle on a metric FAL does not fold down.
The inch receiver (left) (picture #3) has cut outs on the rear of the receiver to allow the dust cover to seat completely.
Metric dust covers don't have tabs on the rear so these cuts aren't necessary.
The locking tab on the front of the inch magazine (left) (picture #4) is shaped differently and is larger than the metric version.
Metric magazines will generally fit into an inch receiver but the opposite is not true.