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Late (self adjusting brakes) have cylinders that have a slot in the piston tops, to hold onto the brake shoes to stop sideways movement which is needed for the self -adjusting mechanisms to work. The cylinders are also "handed" with small bleed nipple on the R/H side and locating roll pins preventing fitment of wrong cylinder in wrong place. The R/H cylinders are still around to buy, but its the L/H cylinders which are rare, and much more difficult to source. I have both in stock.
The early cylinders for manual adjusters, have flat top to the pistons, large bleed nipple on the R/H, and the main "body" of the cylinder is smaller in size. Again, still handed, the small locating roll pin prevents fitting to wrong side.

Thanks very much for your detailed reply Adrian. I seem to be experiencing difficulties getting a dm
to you although I did also send an email from my private email.
Regards,

Bill