This grille surround was purchased on eBay in November 2013 by Edward Vandyk in the UK. It is rather beat up, possibly restoreable. The part is non-magnetic brass. The interesting bit is this tag soldered to the inside of the chrome grill surround. Since the tag is cast with raised lettering, the number RH00307 will be a part number, not a serial number. To the right of the manufacturer's name is "21" which is stamped and may be a serial number.

Serck
RH00307
London

For a bit of timeline on Serck, they also manufactured radiators in a similar time frame. Serck was acquired by the Unipart Group and continues to manufacture radiators today. Serck obviously had a tie up with BMC but was only a trade supplier. They could have been a BMC supplier, as this part would have been available from BMC throughout that period.