A great subject. In fact Shimmels show great variety.
Some were cobbled together much like buying a Track kit, installing the lock and barrel and trigger, cut to a reasonable pull , and sold in quantity to tinkers (frontier salesmen) also selling pots, pans, tools and fabrics who would scrap it a bit, apply a stain (sometimes), a coat of linseed oil, and sell it as a utility gun to back country farmers.
Some smiths, when they were light of customers, would produce unfinished guns, then finish them to the complexity the customer wished. Plainest to full furniture with carving. Then , like now, business was not perfecly consistent and this was how they kept busy.
Some were made this way to order. Dixon once had a fine Boyer target rifle with fine architecture, nicely finished, huge barrel. But with no buttplate and a brass strap T.G. The guy wanted the best to shoot over a log. Basic quality but nothing else. A frugal Dutchman no doubt.
WE do the same today.
