Flak lense has number on outside below O?

[Dastardly_Dan]

So I recently got a pair of Flak's from my in laws. They are customs with OTD lenses and have a number on the outer side of the lense below the Oakley "O" (the hidden O) and was wondering if this is the RX strength? It's 217 on one and 219 on the other. Any ideas what the number is for?

 

[Timhyde74]

WELCOME!!!

 

[ZCForge]

Pics?

 

[KarlBlessing]

Pics would help. Can't imagine they would engrave the prescription on a visible (when mounted) portion of the lens. Other possibility is a molding lot number but that wouldn't be on Oakley lenses (hence the pictures ).