No, the GP510 LR's do not have a stamp.
With these, it may be possible that you received 3.0 by ,mistake, but unsure.
The Joye 510 reg and mega/XL are rated and test at 2.3-2.4
As I tested on the multi and my provari
The GP510 LR I tested are on the money at 1.9-2.2
I have noticed my provari possibly tests 0.1 over, but pretty accurate otherwise.
With these, it may be possible that you received 3.0 by ,mistake, but unsure.
The Joye 510 reg and mega/XL are rated and test at 2.3-2.4
As I tested on the multi and my provari
The GP510 LR I tested are on the money at 1.9-2.2
I have noticed my provari possibly tests 0.1 over, but pretty accurate otherwise.