After a few year of production, the ZB26 was given an overhaul and redesignated the ZB30. The two designs look externally very similar – the only distinctive way to tell them apart is the addition of an adjustable gas port at the muzzle of the ZB30.
Internally, there are enough differences that parts are basically not interchangeable between the two. The ZB26 bolt has two large hooks to engage the bolt carrier, while the ZB30 has these changed to a single central hook on the carrier, much like the later Bren. The barrel lock was redesigned to more positively prevent firing with a partially-locked barrel, and the barrel itself is slightly different. Barrels are not interchangeable between the two guns.
The ZB30 was a bit more robust of a design, and better suited for handling a variety of ammunition types (not that the ZB26 was a slouch, but there’s always room for improvement).
We have a couple ZB30 manuals available for download – a basic one in English and a more comprehensive Spanish one (including ballistics charts):
Factory ZB30 Manual (English)
ZB30 Fusil Ametrallador (Spanish)