If your old Gas cylinder's releaser button area at the cylinder top where it connects into the mechanism (attached to the seat bottom) looks like the photo, that taper design about 3/4" is obsolete and there are no replacements or adapters. A new cylinder will not fit into your mechanism.

Only by upgrading the mechanism will a replacement gas cylinder then fit. It is not necessary to also replace the base containing the casters.

 It is important however, when purchasing any gas cylinder COM assembly, to measure the outside of the column diameter to know whether you require a 45mm (1 3/4") or the common 50mm (2") column which fit's into the chair base center hole.