Our original RMR02's w/4 digit serial number appear more durable than all later RMR's produced after the introduction of the RMR-A's, all of which appear to have 5 digit serial numbers. In a slide mounted configuration, I would not trust 5 digit RMR02 any more than a 5 digit RMR-A until Trijicon unequivocally announces that more robust RMR's are being produced.