Humpty Dumpty says
Functuonally, there's not much difference. Both would weigh about 3.2 kg, have 725 mm barrels choked (unless a rare special order) half and full, not usable for steel, double trigger, ejectors and extractors. I'm talking 12 gauge, but 58 also came in 16 and 20. Technically, though, they are very different. Izh 26 is the classic Anson&Deeley design, with color case hardening and everything, better looks, a bit better balanced. Izh 58 is more modern, actually, it's the same design as the currently produced Izh 43 (Spr 2xx) with top fastener, and many parts would fit; it's a bit stronger than 26, but not as refined.

http://www.shotgunworld.com/bbs/viewtopic.php?t=264728

USAF RET 1971-95 [Linked Image from jpgbox.com]