Those blocks are made for specific uses. There are all sorts of subscriber and central office applications that use 25-pair connectorized cables. They are not generally used in 1A2 installations. The blocks to use to terminate key equipment are 66B25, and the blocks for station cables are 66M50.
It's just the way it is that there are 5 line keys on a 565 set and 4 slots in a 551 KSU. I think you're trying to read too much into all this. These systems are made for growth, and the number of keys versus the number of slots is not correlative. There are 3-line sets (54x series) 5-line sets (56x series) and 9-line sets (83x series.) There are Call Directors that come with 5, 11, 17, and 29-line capacities. There are 584C panels that provide 13 slots per panel, and they can be multipled infinitely.
If a subscriber needed a couple of lines and a manual intercom, a shoebox was prescribed. The original 1A1 systems (301-type KSU's) had line circuits for 4 CO lines, and had a dial selective intercom (207C) unit factory-installed. So in that case, the phones "matched" the KSU, but it was just a coincidence.
When 1A2 was introduced, and a shoebox was too small, then the installer would put in a 501, which has room for a 207C, and a 216A. Some 501 types came with a prewired power unit, and some didn't.
In later years, packaged intercoms and other packaged devices like amplifiers, service observation units, and special relay assemblies were designed to fit between the 7" space between the horizontal bars of a 501 and other cabinets like the 16C.
