One of my copies of AP1086 Sec 6A, this one dated 6th September 1966 to AL/181 has the Mk28 Altimeter as fitted to the P1127. I would hazard a guess that it was also fitted to the later Harrier aircraft as well.
This copy of AP1086 has the sec/ref for the altimeter as: 6A/10558, I would suspect the number you've quoted as being just a part of the Nato Stock Number NSN. A close up pic of the label would make it clearer.
Hope that helps.