Mysterious ":h" or "4:" AR15 upper markings

[ReluctantGlockFanboy]

This PSA upper I just got doesn't have any of the normal forge markings above the forward assist but has this weird :h or 4: (depending on how you look at it). Anyone know who this might be forged by?

More for my curiosity than anything else. It is 7075 and seems well machined based on my caliper inquiry.

 

[Button Pusher]

Perhaps a mark to identify the CNC unit it was machined on.

 

[CITYPUNS]

This PSA upper I just got doesn't have any of the normal forge markings above the forward assist but has this weird :h or 4: (depending on how you look at it). Anyone know who this might be forged by?

More for my curiosity than anything else. It is 7075 and seems well machined based on my caliper inquiry.

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The letter or number inspection stamps for qc are day vs night shift indicators this may be the same thing - former employee

 

[Stogies]

The letter or number inspection stamps for qc are day vs night shift indicators this may be the same thing - former employee

Claims were made PSA has no QC? Just joking.