There was some inconsistency over the German numbering system. They were restricted in developing new armaments after WW1, so up to the mid-30s often gave a spurious number like "18" to indicate that it had been developed during WW1 and was therefore permitted. In fact, the 8,8cm Flak 18 entered service in 1933, the Flak 36 was a modified version which came out three years later, the Flak 37 a further modification.
There were two very different 8,8cm Flak: the 18, 36 and 37 family all fired the same ammunition, the 41 fired much more powerful ammunition giving it a greater effective altitude.
The basic figures are these:
8,8 cm Flak 18, 36, 37: 9.4 kg shell at 820 m/s, effective ceiling 8,000 m
8,8, cm Flak 41: 9.4 kg at 1,000 m/s, effective ceiling 10,675 m
10,5 cm Flak 38, 39: 14.8 kg at 881 m/s, effective ceiling 9,450 m
All data from Hogg's 'German Artillery of WW2'.
Tony Williams: Military gun and ammunition website and discussion forum