I am not an authority, so this isn't worth much. However, since you have no answers I will venture that, yes, you can shoot steel in them but not larger than 4s. Exp in the Arrieta.

These are the instructions I got with my Merkel made back in the early 90s.

You may be able to load it with larger shot and be safe, but w/o specific instructions (have you tried the importers or makers?), that is all I would do - 4s max.

Brent

_________
BrentD, (Professor - just for Stan)
=>/

[Linked Image from i.imgur.com]