It might be one of the Stoeger imports that were imported in the late 1960's and early '70's.
Those imports weren't known for their high quality but if it has Belgian proof marks it should be safe to shoot.
Your best bet to determine the thread size is to measure it.
If you don't have a precision dial caliper or micrometer the folks at a local auto supply should be able to tell you what the thread size is. They will have a thread pitch gauge to determine the thread spacing.
The threads might be metric 5.5mm, 6mm, or #12, 1/4", or 5/16" in diameter.
Threads vary but are often .75mm, .9mm or 1mm in metric pitch or 24, 28 or 32 threads per inch.