Absolutely great research, Alexander!
I would never have generally ruled out the early existence of iron ramrods for heavier pieces, and I am glad to be able and prove your discovered contemporary artwork by two really existing wrought iron crudely sighted haquebut barrels, both possibly Nuremberg made, about 1500-10, ans struck with a maker's mark, a shield with some pellets, the swiveling touch hole cover of one missing, the latter barrel heavily damaged and broken in two in the middle, and both retaining their iron ramrods characteristically bent outside the muzzles in order to prevent them from going in too far - just the way Alexander found them illustrated!
Both are preserved in the Museum of a small Northern Bavaria country town named Wunsiedel, where I took photos of them amost 30 years ago!
As 'Nando pointed out, I would however like to reemphasize the fact that iron ramrods were the exemption to the rule until the early 18th century. The Landeszeughaus Graz preserved hundreds of separate thick wooden ramrods mounted with iron finials threaded for screwing in cleaining tools for their mid 16th c. wall guns (
Doppenhaken), and the huge 35 kilograms of weight Nuremberg bronze Doppelhaken of ca. 1520 in my collection (posted here earlier) also retains its original fir wood ramrod (repaired in places), its iron finial retaining its original cleaning tool.
Best,
Michael