Got a pair of spring loaded back-ups for you here, Magpul MBUS (I’m going to say ‘style’ as I am unsure if they’re gen Magpul, or licenced replicas). As they are flip up, there’s no concern about co-witness, just pop them out the way, and if you need them, they’re Spring loaded and will stand to attention quicker than your ‘sgt major’ in whore house filled with porn stars. 
 

Used, so they’ve seen some action, sadly not the kind referenced above, but they’re is good, almost as new condition. 
 

Yours for £18 inc postage.