Quoted:
The marking are there to state the units EOTech sells are suppose to be sold to military and LEO only.  Bushnell is the official civilian retailer with the Holosight.  What EOTech dealer do with their Model 510 and 550 is beyond the control of EOTech.

500 series which is the civilain model, it is slao sold under the Bushnell banner is the basic model. it only come with the hex screw, and it has a build in auto shut down of 8 hours.

510 series is the tactical model which has a protective hood, thumb screw, a bit different electronics and it has a programable auto shut off of 4 or 8 hours depends on the switch you hit when you trn it on.

550 series is the military model, it is the 510 series plus it has the new upgraded rubberized buttons, NV compatible, and it is dive capabile to 33 ft for 24 hours and 66 ft for a undisclosed period.