This isn’t an office building, and nobody “hid extra pairs in the wall” for good reason. This wiring is a very specific technique used to quickly feed phone pairs in new SFH and apartment buildings of a certain era when the minor CAPEX advantage of a single shared cable over home runs was sold to developers as a cost cutting measure (or just done anyway to pad wallets when home runs were specified).

You’re definitely right that it uses standard 25-pair color code, but it’s just a six pair residential run here: white from blue to slate, and red-blue.

I’m only seeing 6 pairs here. And, as I recall, 8 pair ewire is not a thing. 6-12-25 are the standard sizes for single binder in-home wiring of the era.

Little high on that pair count, look again. 😉