From Confidential Service Memorandum MG/264 dated 24 July 1959:

CARBURETOR DAMPER PISTON 'MGA' TWIN CAM

An improved carburetor damper with a shorter piston was recently introduced (PMG/347).

The original dampers (AUC8103) could cause uneven running and a restriction in maximum speed but are interchangeable with the new ones (AUC8114) giving an improved performance. Alternaterly, the old type can be converted to the later specification by machining .070" (1.78 mm.) off of the lower face of the piston.

The drawing shows the later type damper 'A', together with the dimensions of the new piston 'B' and the old piston 'C'. Carburetors with modified damper pistons should have the hexagonal cap stamped with an 'O' for identification purposes, as shown at 'D'.