Solothurn S-18/1000. Both 20mm x 138mm belted caliber.
Comparison: Finnish Lahti l-39 Norsupyssy on the right, Solothurn S-18/1000 on the left.
Thanks, that was nice finding a pic with both in it. I think it was ‘96 or ‘97 when I read the book. (where the heck did the time go?)