I can confirm that post-war Wather Röll rifles were 126- serial numbered and also stamped "W.Röll", or similar, (on the barrel, but hidden inside the stock). By contacting the proof office at Ulm, (who stamped the antler proof mark and date), they should have the original record of the serial number and, most importantly, who submitted it for proof. Perhaps Peterlongo purchased the action afterwards?

Here's some notes I've saved:

W. Röll made that rifle for a French officer circa 1946/47.
The 2nd DB of Leclerc (1) occupied Oberndorf first before French put their own administration at head of the works, Walther Röll and Walther Gehmann choosing freely to work with the new occupying forces.
During that period many sporters were made for French officers, using disparate parts in stock, and amazingly the firm accounts department took great care to invoice every rifle ordered by Frenchmen for their own use !

In 1942, before the battle of Stalingrad, the Nazis still felt to be future winners of WW2. Mauser, Oberndorf never stopped making civilian sporters. According to Jon Speed's "Mauser-Original Oberndorf Sporting Rifles" they made about 1300 sporters in 1940, 1000 in 1941, 250 in 1942, 350 in 1943, 155 in 1944 and 135 from 1945 to 1950 under French occupation. Later, Walter Röll, until 1946 head of the Mauser commercial shop, made at least another 1600 Röll-Mausers from leftover and new-made parts until 1965. (Kuduae 11/2013 Double Gun Forum).