Yep, the manual is wrong, most 1275s, as Gerard staed came with the stamped steel rocker arms, only the very first ones had the forged units, and they work just fine, they were two varieties of the stamped units and both work fine, the ones with the Vandervell markings on them, than the ones with the PATD stamped into them, they claim these have a tick more lift.
FWIW, the sintered rocker arms came on the later A+ plus Mini engines.