I had recently acquired a CO2 cylinder which has the below marking and after some on-line research I found that the meaning is as follows:
CO2 - 2 TP250 V3.0 W6.13 BXXXX XXX 2011/03
Marking Meaning:
CO2-2: Filling gases - Carbon Dioxide
TP250: Test pressure - 25MPA
V3.0: Volume - 3.0L
W6.1: Weight - 6.1KG
BXXXX XXX: Product No.
2011/03: Hydrostatic Test Date - CO2 cylinders needs hydrostatic testing every 5 years