John Sneddon's book says:
"March 1971 Car number YG2S4 2918 - Rear Hydrolastic Displacer units Part number 21A2012* replaced with AYG4113"

That was the 2nd last Mk2 S made according to the same source.

In UK the hydro suspension was replaced by dry suspension on 1275 GT in June 1971 meaning the supply of the silver coloured displacers would probably disappear soon after - assuming Aust sourced S and GT units from UK.
If Zetland wanted to finish the GT production run with hydro units, could they have had the last ones made in Sydney, as their new part number implies. This might explain why late GTs have displacers without stamped numbers and with colour codes that are not commonly listed in the usual references?

*21A2012 was the front displacers, usually. Should be 21A2014 for rears.

My bags are 2/67 and from what I can see from above they have no numbers stamped or branded into them

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1960 Morris Mini Minor

Do you mean your car was made in 2/67? Is it an S or De Luxe?