"All the stampings were professional "one-line" stamped and not "single-letter" stamped.

Would this indicate that the conversion was done by a larger retailer / builder as opposed to a basement gunsmith?"

Not really.

IF the conversion was done in an EU country or a country where guns have to be proofed (as opposed to the US), then
it should have reproof marks or symbols on it - as well as the info you said is already stamped on it.

Each country uses different symbols for proofing so no point is listing them all. If UK done, it should have the new load in Lbs per Squ Inch or something like that.

A photo of the barrel flats would be best - would take 2 minutes to sort it out.

Re calibre conversions, not a problem if done correctly.

A lot of Rigby's were done in the 70's and 80's to 9.3 and 375 Flanged Magnum. I have one.
Can't think of the guy who did it but he now builds Doubles under the Jeffrey name and used to under the Rugby name.

If the conversion was done in the US, I would like to know who. If someone like JJ at Champlin Arms, all well and good.

I am sure others will be along with more advice.

Can you get a photo ?

or a link to the gun if on a web site ?