2nd class blue Machin security stamp counter sheet 2017 version M17L (Number corrected from 2911.7 to 2911.7a)
The 2nd class counter sheet was reprinted on 04/01/17 and appeared in February.
A further reprint was made on 22/03/17:This is the first printing on Security Backing Paper (version 2 – SBP2)
Probably SG U2957 (also used for earlier years).
Cylinder blocks of 6
Date block of 8 - 22/03/17