What other cookies/biscuits were traditionally baked in shell shaped forms like this one?
It’s not a perfect match1, but considering the region and that you apparently have just one or a few, I would assume that this is a slightly unusual mold for “Bärentatzen“ („bear paws“), a classic Swabian Christmas cookie.

(Source)
A soft but stable flourless dough made with whipped eggs (or just egg whites), sugar, chocolate and almonds is first shaped into balls, coated in sugar, and then pressed into a wooden or metal mold just to shape them. The sugar prevents them from sticking and they are baked not in the mold, but free on a baking sheet. If you look closely at your mold, you’ll see that there is none of the slight polymerized grease residue that suggests that it was ever put into an oven.
Molds are traditionally made of wood (“Model”), usually beech or other hardwoods, but I have seen tin ones as well - the metal ones were more common in the earlier 20th century (pre-WW I&amp;II and shortly after), but often had problems with the material suffering over time. Wooden ones are seeing a comeback in recent years.
Bärentatzen are pretty local - older Swabian cook books have them2,3, in the rest of Germany they are virtually unknown or mean another type of cookie closer related to spritz cookies. Food historians have suggested that the original shape was supposed to be scallops or seashells (think the religious symbol of the pilgrimage) and that the reference was lost at some point. The alternative name „Schokoladenmuscheln“3 („chocolate scallops“) is another indicator.

1 Typically Bärentatzen would be single shells. The baking equipment company Staedter sells a wooden version of a double shell  and of a single shell as „Spekulatius mold“, which is in my opinion imprecise as Speculaas are traditionally very flat and at least the single one is sold as „Bärentatzen“ by Birkmann, another baking equipment supplier.
2 Kochen und Backen nach Grundrezepten, Luise Haarer, 1965 (and others)
3 Kiehnle-Kochbuch,  Hermine Kiehnle, 1928