Here are two varieties of the CD 164 Hemingray-20 over 42:

The SDP piece on the left is closest to the [100] embossing. However, note that the '20' is simply engraved over '42' (they did not bother to blot it out first). I have not yet seen one with 42 blotted out.

The one on the right is a currently unlisted RDP version, very generously given to me by Rob Bachand ([id=747652239] for a closeup of the stamp). This one is also over-engraved and not blotted out.

They are different molds, so the engravers messed up on more than one occasion. I would guess they caught the error and fixed it right away. You never know though, there could be an uncorrected CD 164 Hemingray-42 error out there... keep your eyes peeled!