I recently bought a Falco S/S .410 It came with swivel fittings already on. As I like the removable Uncle Mike type swivel fittings I sent for some of these. Bought a very good thick leather cobra strap at the Midland.
Yesterday I removed the rear swivel on the stock and put the new fitting straight on as it was the same wood screw thread. Then I came to unscrew the one in the fore end. It had a machine screw thread, which I thought was a bit strange. As I tilted the gun to look at the hole the fitting came out of, the fore end came off in my hand. The machine screw held it on and went into a chunk of metal between the barrels. As the new fitting was a wood screw it wasn't going to work. The new leather strap was too wide to fit in the old swivels. Bother and other words were said.
Got OH with his trusty vernier callipers to measure the old swivel and it was 3mm thick on the strap loop. The New removable one was of course 3.5mm with no play in the swivel to fit it.
With care the thickness of the existing screw head was measured and OH decided it could take another 0.5mm. The fixed swivel loop was cut away and a larger diameter hole drilled. New removable swivels successfully fitted and strap attached.