Not 30.0x...
Emphasis is mine...
https://www.law.cornell.edu/uscode/text/18/930
The posted sign would reference 18 USC 930... At national parks, the signs look like this.... But I've never seen any similar sign at my local PO.(h) Notice of the provisions of subsections (a) and (b) shall be posted conspicuously at each public entrance to each Federal facility, and notice of subsection (e) shall be posted conspicuously at each public entrance to each Federal court facility, and no person shall be convicted of an offense under subsection (a) or (e) with respect to a Federal facility if such notice is not so posted at such facility, unless such person had actual notice of subsection (a) or (e), as the case may be.