What is the difference in the L1A1 , R1A1 and the STG58?

L1A1's are inch pattern. They make up about 10%
R1A1's are metric pattern. South African. Generaly in poor condition. They really beat them to death. I don't recomend.
STG-58 are metric also. Made by FN or Austrian. These are the best kits. The Austrians took good care of there guns.

Mag

'R1A1' is a model designation used by Century for its metric pattern receiver. Century has used this model marking on imported IMBEL receivers and its own U.S. made receiver.
Century has used the 'L1A1' marking on its inch pattern receivers.
'L1A1' is also a general term to identify the family of FAL rifles made by Great Britain, Canada, and Australia.  
A receiver does not necessarily identify of what type or pattern are the remainder of the parts on the particular rifle. Inch and metric parts can be combined and be part of the same rifle.
'STg58' is an Austrian model of metric FAL rifle. Parts were made in Belgium and Austria.

Harlan
FAC