According to the DMV, custom collectors' plates are prefixed with HR and run from HR 100 - HR 999. These plates seem to be described differently from historic vehicle plates, but I'm not sure how they are different. The prefixes for historic vehicles are HV, HW, and HX, which seem pretty close to HR, so they are clearly intended. to be close to each other in some way, but they are definitely different.
Custom collector's plates are governed by these statutes: