Weird pagination

I've been reading a bit in an old encyclopedia, Encyclopedia Americana, from around 1965 and in volume 15 I noticed that page 26 was followed by 26a, 26b ... 26z, 27, 27a, 27b, ... 27z, 28, 28a, ... 28n, 29,30, ...

I've never encountered that kind of extra pages in books before. Any idea what was going on?

My guess is that most of the printing was done with the old plates, but here and there additional articles, or material in existing articles was added.

And didn’t some encyclopedias have an index volume?

>3 2wonderY: Spot on. Volume 30 in this encyclopedia is the index. Maybe they planned ahead for this, since it is some of the large subjects like "India" that is 60 pages but just spans from page 29 to 32 :-)

And yes like @MarthaJeanne suggests there must have been earlier versions and they reused a lot of the old pages without changing them, so new or expanded material created problems with the pagination.

I'll add something to the pagecount in LT. I wonder what librarians do in such cases. Using the last pagenumber in the volume is off by 100 or so :-)