Not all Gibsons are the same. 498 bridge version are wider spaced than a 490 neck version. Both sit perfectly under a string. One of the few improvements that actually achieve something useful.....except I hate the 498 anyway!
There are 2 spacings I am aware of that are commonly used. One is called F-spaced which would be a bridge position pickup which the poles end up centered directly under the strings. The other is just the common pickup spacing. Gibson and most others just use one spacing for both and the poles are not directly under the bridge strings but skewed a bit to the outsides of the poles. Lollar has some F-Spaced and I am sure others do as well. I have several Collings with the F-spaced. No noticeable difference to my ears.
51mm. Is your Ooglie broked?
'89 could still be both pickups spaced the same.
Would either be the PU490 (80's T Top basically), or the HB-L and HB-R, (circuit board) which I'm not sure if the pole spacing is wider on the bridge variant.
Humbuckers started being made different widths long ago because they tend to sense vertically. It has only been recently that P90s started being made different widths for neck and bridge because they are more sensitive horizontally. It isn't so important if P90 widths are the same for neck nd bridge. Humbuckers and P90s have different shaped magnetic fields.
Typically humbucker neck and bridge pickup pole widths differ 2mm.
WTF! No that is wrong and shows a complete misunderstanding of pickup design and magnrtic field/string interaction. Throw you book away. The very simple reason for pole piece width change was to place the poles under the strings. The pickups are the same width, regardless.