I concur with Ken that Wheaty's gun (and mine, also marked "Zbrojovka Brno") are likely postwar. Wheaty's is noteworthy in that it shows some later Britishgenes: the earlier AA sight flange on the rear of the receiver is absent. However, the flange is present on mine. Possibly both receivers were leftovers found at Brno after the war. All the postwar numbers I have seen were, as Ken points out, in the 2xxx range.
Peter: I think I confused ZB39 with ZGB39, the former being the Bulgarian 8x56 gun made only by CZ c. 1939-40. I have a parts set for one that was imported from Bulgaria but surprisingly has Waffenamt-63 proof and acceptance stamps on the barrel only (not on any other parts).
Also, the mention of a "castellated" nut in the trials reports you referenced is odd, as the barrel nut is not castellated, but the flashhider is. And it is threaded, in prominent contrast to the Mk1 Bren. Just a thought...
M