Original LH Torana 253 engine number prefix is HR (H for LH and R for 253), so I reckon yours is (as HK suggested) a police issued number. They were done when a block was decked and the engine number removed. If it wasn't re-stamped by the engine reconditioner it usually was just left blank until the car was inspected for rego, then the relevant Motor registry would require a Police number for rego. No big deal, but it screws up the history of the block.