The Heeren action was the invention of count Arturo Massa y Heeren, a Spanish-German gentleman, in 1880. He gave the patent to Nagel & Menz in Baden-Baden. So the early Heeren actions are all marked by Nagel & menz. But N&M never made "their" rifles. As usual they had them made by the Suhl guntrade, first by Stephan Heym jr., then by the son Max Heym. Both are not related to the still existing F.W.Heym company, now in Gleichamberg. After the patent expired the Heyms, father and son, made the rifles under their own name. Max Heym had the word "Heeren" protected as a trademark. Max Heym continued up to 1938, when he sold out to Ernst Kerner & Co. because of old age. Kerner continued to make Heeren actions and rifles until 1945.
IMHO rechambering this rifle to .25-35 Winchester is a perfectly good idea.