Yes, there are more screws to remove: You will find one or two metal to metal screws on the underside of the action bar, in front of the triggerguard. Remove these too. Only now you should be able to remove action body and triggerplate from the stock.
Many German break open stocks were ruined by GIs in 1945 who tried to remove the stocks by forcefully pulling them out to the back like on a Winchester repeater.
BTW, a post-war GDR Simson looks exactly like a Merkel because both were the same then. Neither the Merkel nor the Simson companies existed any more. Both were “Socialised” and incorporated into the VEB Ernst Thaelmann Werke (named so for a communist "marthyr"). The ETW used the old respected names merely as brands for their products. As the Simson family in the USA still had the rights to their name, the ETW branded all their guns “Merkel” when destined for export to America. For export to other countries they branded all side by sides “Simson” and all over unders “Merkel”. The names did not matter: All those guns were made by the same men in the same factory.
That “1st Quality” mark is not related to the individual gun. Those quality grades were assigned to products of all sorts, not only guns but optics and even pickled cucumbers. It merely said: These ETW model of gun is a high quality product, fit for the world market.