The near rifle looks to have had a neat patch over the site of the volley sight plate (the machining for the rear volley 'arm' is still evident). I suspect it may just have been a unit that had flaps to hand & so fitted them to aid in keeping the bores free of dust/muck. But even this wouldn't have been an ideal solution; the round dimple on the Flanders Flap is of a size intended to sit in the muzzle of a 303" calibre weapon, & these, on an indoor range, are likely to be No2 MkIV*'s....aren't they?..........Odd.