KDGS,
The Garcia guns are as you suspected and are equal to the earlier S3EL and S3EELL designators. The difference in engraving from the SO4 to the SO5 is very minor. They both have hand detachable sidelocks and weigh about the same at 7lbs and a little. The SO4(1970) and S3EL(1953) are engraved by the same man(Angelo Baglioni) and have the same coverage. The guns made for Europe may have been designated differantly than the Garcia ones. I don't have any personal experience with the SO guns outside the US. The SO guns have many variations and thats half the fun.
Best,
Ron