It get's harder to find the Mk.1 version with the square shaped clips. However, I had a passenger side unit to hand and apart from the clip being backwards it wouldn't run out smoothly because the inertia mechanism locks when tilted left as it's used to sitting right.
I took the old chewed unit and drilled out the star shaped screws with a strange head in the middle that hold the plastic cover ( make sure it's the right cover because the other contains the spring which is just waiting to leap out and un-coil), thus gaining access to the mechanism which is essentially a ball bearing in an angled tray which presses a lever onto the ratchet depending an it's angle. The left side belt has a black tray and the right a white one. Drilled out the other belt and swapped the trays and replaced the mechanism cover with normal screws ( the even earlier belts didn't even have these weird security screws but four rivets) and it works perfectly.
I hope no-one has to do this but it saves having to have matched pairs or if , after removal from scrap vehicle, the belt has returned too far ( always pull it out and knot to prevent) and you have to activate the locked lever.
Blimey, I always took those things for granted and now I'm a seatbelt bore
