The black/white tabs are the same between an AMT/510 and STGW57/PE57 so you should be able to find one. The purpose of the tab is to restrict or allow full auto fire. When the white side is visible on the outside, the interior black side will not allow the selector to be set to the full auto position. When the black side is visible on the outside, the interior white side will allow the full auto selector setting. So for the OP's weapon the tab should show black and not white.
Here is a STGW 57 grip showing black tab:
Here is a comparison of a grip tabs:
Notice the size difference between the white side and the black side. The longer black side when on the interior will restrict the selector movement to the full auto position.