OK, H-D Co. changed the method for producing the upgraded rear hub w/ -55 #. Looks good to me, and that's why the number became -55A. Correct ?
i can clarify the 55-78 rear hub part number questions and descriptions.
hd lists the hub in two states of assembly
bare hub
or
hub assembly (includes bearings, seals, and shields)
there are 2 pn's for the bare hub and 5 pn for the complete assm.
the bare hub:
41018-55. the "thin" hub used 55-65. to my knowledge only one construction. (we need pics of the mystery hub interior side of drum flange to get a better idea if that hub was factory or a repair of the common thin flange failure).
41018-66. the "thick" hub.used 66-78. two constructions brazed or welded. this hub has a thicker flange on brake side, it also uses longer lug bolts than before.
complete hubs:
41017-55
used 55 "only" (see -55a. this assm was reintroduced to in L58 as pn 41017-55a)
its the thin hub fitted with 9010 brake side bearing.
41017-55a
the thin hub fitted with 9011 brake side bearing.
used 56-e58.
in late 58 this -55a pn was retained but the part you got was the earlier -55 assm. ie: the thin hub fitted with 9010 brake side bearing.
used L58-62.
41017-55b
its the thin hub fitted with two bearings on brake side. one 9008 and one 9009.
used 63-65.
41017-55c
its the thick hub fitted with two bearings on brake side. one 9008 and one 9009.
used 66 to around e74 as brazed construction, then welded till 77.
41017-66a
same as the welded -55c.
part number changed for the 78 model year.