SR plates are 'Special Registration' for some time people believed that these were Street Rod registration as they are controlled under the Australian Street Rod Federation with delegate Con Manzaris as the nominated person in control.
The SR plates mainly appear on Street Rods, although I have seen one Cobra with them. You may make an enquiry with the Federation to gain Con's details. Con lives out Dandy or Doveton way and may be in the telephone book..
CH plates are Classic Historical plates. They are under no one single persons control, but are however controlled by a number of clubs that have approval from the rego authorities.
Perhaps you could make some enquiries with the authority to gauge which direction may suit you.
Both plates provide limited use, however this is being relaxed somewhat. The rego people appear to be very focused on stopping owners from hiring cars.
Again check the site out.
See you at a meeting - if I get there..