The "standard drop timing test" in the SU shop manual is to remove the suction chamber, damper assembly, & piston, thoroughly clean with solvent. Temporarily PLUG the piston vacuum hole with clay or some such. Assemble the piston, damper, & chamber dry. Invert the assembly, push the housing fully up on the piston, & measure the time for the chamber to drop off the piston. Don't let it hit the floor. Standard specification is 5 to 7 seconds for the chamber to drop. A slightly dented chamber wall could cause binding & longer drop times, as would a bent/binding damper rod assembly. Air leakage around the damper cap could cause faster drop timing.
This measures the air leakage rate between the piston & housing which is an indication of how closely the piston is clearanced to the housing. Not particularly important on single carb installations, but very important when matching 2 or more carbs.
In your case, it does appear that one piston may slightly undersize. Other causes of different drop/lift timing under ACTUAL operation could be a metering needle binding in it's jet, check jet centering, or a restricted vacuum passage in a piston.
The early MG carbs which have no return springs can be especially critical in this area. Adding the lightest available return springs "might" help but should not be needed.
If there is "adequate" oil in the damper chambers, level slightly below the inner chamber top, oil levels should not be critical. Both damper sleeves should have a bit of free float. They are actually a one way valve which restricts piston rise but allows it to drop freely.
D