From my memory (dodgy) local parts suppliers only stocked 1 size and it was 0.100". No idea what part number it was sold under. They seem to be mild steel - only ever in compression and not a bearing surface.
According to this attached excerpt from Leyland Special Tuning for raising ride height on Cooper S and 1275GT, there were 4 sizes available - 21A1845 is the thickest at 0.150.
Attachment:
Ride Height Leyland ST.jpg
I am not aware of any spacers being used on the rear struts as the rear usually sat higher than the front and needed pulling down with stronger helper springs.
Note the statement that packing washers can be used providing the drive shafts remain horizontal, implying that the ride height is set to achieve it.