pre-WW2 Czechs-made links are bare steel devoid of any surface coatings. Complete and loaded belts with such bare links are regularly recovered by EOD personnel in the region of the Atlantic Wall in France (the mod. 30 Czech weapon was commonly used as anti-aircraft by the Germans on the Atlantic wall).
The first mod. 30 Czech Maxim was intended for aircraft fixed mount. Therefore, the first links for that configuration was based on the original WW1 Prideaux design with square corners and permitting almost no fanwise and helicoidal flexibility. These first links are bare steel.
1st model of Czechs links for fixed mounted Maxim mod. 30 :
When the weapon was adapted for aircraft flexible turret mount, it was required that a new link be designed in order to increase both fanwise and helicoidal flexibility. Therefore the links were modified by increasing the internal diameter of the single loop (from 11 to 12mm) and chamfering the top and bottom connecting wings (b).
Here is the comparison of 1st model (left) and 2nd model (right) :
Here is the illustration of the fanwise flexibility of the 2nd model of links :
I have only observed the second model of link bonderized as produced during WW2.
I would be very interested to see pictures of the blued links that you identify as Czechs pre-WW2 production.