I dug out the Swede pseudo-scout to check the fit of the T/C rail. If anything, it looks like the rail diameter is slightly smaller than that of the Mauser barrel. The join is tight at the edges but has a slight gap in the middle. I suppose it's possible the gunsmith put a new radius on the underside of the rail and didn't tell me about it. The guy was semi-retired, and tended to do things on his own recognizance without consulting me.
At the time I built the rifle (ca 1999), the "Thompson rail fits military Mausers" bit of information was fairly common on the internet. It seemed to work on my rifle, so I never questioned the notion, or looked at it closely until now.
Here are some photos. The barrel-to-rail join was hard to photograph with my cheap camera, and the glossy barrel didn't help.
The rifle was built on a sporterized Swede Mauser carbine. Generic Thompson rail from Brownells. Ashley Outdoors (now XS) rear sight, with extra tall Swede Mauser blade in original front sight base. Bell & Carlson Carbelite stock. Timney trigger. Buehler safety. Vintage 1999 Andy Langlois sling. The paracord wraps are to keep the sling from rattling against the swivel loops. The gunsmith converted the floor plate to a hinged model, and narrowed the trigger guard. He also gave the barrel and receiver a mirror polish, which I did not ask for, but didn't have the heart to make him bead blast. (These days I ask a lot more questions up front, and am less tolerate of unasked-for changes.) Weight is a hefty 8 lbs.
Clearance between scope bell and receiver ring is a bit over 1/8". I originally had lower rings on it, but the scope actually touched the receiver. I could probably get a better fit by experimenting with a different brand of ring, but I rarely use this rifle so it's not a priority. Also, I just noticed that the rear scope ring is too far back.
The sharp eyed observer will notice that the safety lever is missing. The screw holding it on had become stripped and fell off during a hunt. It is destined to be replaced with a Mauser wing safety.
If I were going to do it again I'd leave the original safety and 2-stage trigger alone. I'd also pick a different stock. The B&C fore end flexes with sling pressure, and the stock feels bulky around the wrist. I'd also go with a 20" barrel. The Swede seems to lose a lot of velocity in a 16 1/2" barrel. While the Thompson rail works well, I think the rifle would be a lot closer to the mark with a light, thin barrel and a different scope mount. A bone stock Ruger Ultralight or Hawkeye Compact with a conventional scope feels a lot more scout-like than this Swede.