Abstract: A tamper-proof nut assembly for interlocking together the separable parts of a mounting support for a meter for monitoring the flow of liquid from a storage supply thereof to prevent separation of the meter from its support so that it is rendered inoperative to record and register the amount of liquid passing through the supply or delivery line with which the meter is associated, the nut assembly including an elongated tubular member having axially alined integrally formed first and second parts each externally fitted with a revoluble sleeve, one of said parts being adapted for threaded engagement with a bolt for locking together the separable parts of the meter support and the other of said parts being of a polygonal external shape for application of a tightening wrench thereto, the revoluble sleeve which encases the polygonal part of the nut member being removably held in place by a key-actuated tumbler cylinder fitted in the bore of said polygonal part of the nut member.