The visor you have is actually a visor for a US Navy MK-IV full pressure helmet. The way to tell is that the Mercury visor has a semi-circular cutout in the lower right hand side of the visor (as worn on the head) to provide clearance for the helmet oxygen exhaust hose connection. This visor does not have that cutout.
(Note that the cutout does not show up in the photos you have of Glenn in a mercury helmet since it is on the side of the helmet away from the camera that took the photo).
It should also be pointed out that MK-IV pressure visors were made both with and without the stainless steel bands on the edges of the visor. Your visor is of the type with the stainless bands.
Another way to tell the difference, is that the left hand pivot hole in your visor has a small radial notch which allows the visor to be locked in the "up" position. The MK-IV helmet has a tab on the spring loaded visor latch which fits up into the notch to lock the visor in the "up" position. The Mercury visor pivot hole does not have the notch but a radiused cutout since the Mercury visor does not lock in the "up" position but stays in the "up" position from friction from a spring loaded plunger on the radiused cutout in the visor pivot hole.
This visor is still a desirable item, especially if someone has a MK-IV helmet with a broken visor, but it is not a Mercury space helmet visor.