Which Heym, please? There were no less than 60 Heyms active at one time or another in the German guntrade. Even in 1945 the US Army listed two gunmaking "factories"/companies: Friedrich Wilhelm Heym (post 45 in west Germany, now Gleichamberg) and Gebrueder Heym.
Please, post photos of all marks on and in your drilling, under barrels, on the action watertable and maybe on the breechface. Then there may be a hint to the real maker.