Maryland Code, Criminal Law § 5-807

(a)(1) The State need not negate an exemption, proviso, or exception set forth in this title in:

(2) The burden of proof to establish an exemption, proviso, or exception is on the person claiming its benefit.

(b)(1) In the absence of proof that a person is a registrant or holder of an order form issued under § 5-303(d) of this title, the person is presumed not to be a registrant or holder of a form.

(2) The person has the burden of proof to rebut the presumption.