Well, as usual, here is my two-thousand and sixty-five cents worth. If you read the whole post, it might be worth three million dollars when you are done with the economy the way it is. Grab a pillow:

584(X)panels were 1A2 components that provided 13 slots for line or intercom cards. These were mounted on 23" racks and required either a single 75 pair cable or three 25-pair cables to connect to the stations and lines. The power connections were completele independent and usually a part of the rack itself.

551A was a Western Electric 4-line KSU in a metal case that was hinged on the right side, circa mid-1960's. The term "shoe box" was appropriate because that is about the size that it was. It had a special Twist-Lock connector for the power cord to connect.

551B was basically the same thing, except they eliminated the Twist-Lock connector for the power cord. In the early 1970's, a plastic grommet was installed in the hole where the power connector would have been and a simple extension cord was used for power.

551C was the first major change in probably ten years of production. It was 100% plastic in design and had a lift-off cover instead of the former hinged on the right side design. This KSU was a lot more modern in design since most factory connections were concealed behind the backplane. Earlier A/B versions had all card slot wiring exposed and presented the potential for failure due to accidents (many wire-wrap pins in close proximity).

Perhaps the most notable change with the C version, asside from appearance, was the fact that each line card slot had a socket with a standard "51A" lamp (light bulb). This was to indicate the line status for service people. This was probably because the line cards that were in current production (400D) at the time didn't give this indication. Later -F, G and H series cards included and LED to provide this right on the front edge. Despite the innovations and modernization with the 551C, it still used the same power supply unit, line cards and interrupter as the A/B versions.

None of the 551 KSU's had intercom capabilities other than simple "buttons and buzzers". A special line card (401A or B), referred to as a manual intercom card, could be used in any available card slot to provide a common talk path (party line).

External intercom units were frequently used from manufacturers like Tone Commander, Teltone and Melco in systems where dial intercom was required.

Keep in mind that during the time when these systems were being installed, a second or third line was considered luxury, even in a business. There was usually a spare slot for the manual intercom card. Today, wasting a CO line port for this purpose is unheard of.

It's important to know that Western Electric recycled/refurbished just about all of the hardware that the Bell System used until it couldn't be used any more. For this reason, a "state of the art" circa 1982 KSU could easily be equipped with circa 1964 line cards. The design of the 551C was probably intended to extend the life cycle of refurbished parts. That's just my take on it.

The ITT 601 was similar, yet entirely different if that makes any sense. In most aspects the functionality was the same, but all cards were mounted horizontally on the left side of the cabinet. I think that they may have been on the "cutting edge", as in more than two or three lines. The 601 supported five lines AND intercom.

That never made sense to me since the most popular 1A2 phones (2564/2565 six-button) couldn't handle that capacity. To do this the 2830 (10-button) sets would be required, but even with intercom, this KSU would land the customer with three dead buttons. Despite the remarkable flexibility of 1A2, I just could not understand some of the configurations of the basic KSU hardware.

The ITT K601 had a lift-off cover like the WECO 551C, but it was a whitish/gray color where the WECO unit was simply gray.

With the 601, the uppermost card slot contained a solid-state power supply which was proprietary, as in it could not be replaced with generic parts. An optional ring generator could be screwed into the KSU's base and plugged into this card. Below the power supply card slot was another dedicated slot for yet another proprietary card that provided the interrupter functions. Unlike the 551 systems, the 601 used a simple 400-series card as the interrupter. For those unfamiliar to 1A2, the interrupter simply is a device that creates various lamp flashing patterns and ringing cycles.

Below the interrupter card position were slots for standard 400-series line cards. This KSU, as with the 551 or any other 1A2 system, could use any manufacturer's line card. The 551 only supported four line cards, yet the 601 supported five.

Below the last line card slot in the 601 KSU, there was a specially-wired slot to accommodate a plug-in dial intercom card. The basic card supported rotary dialing only. A piggyback card could be added to provide touch-tone receivers to allow tone dialing on intercom.

Both the 551(X) and 601 KSU's were designed to make it simple to install. Virtually every connection was pre-placed so that the installer could literally punch down the appropriate number of pairs in color-code order from the 25 pair station cables and it would work. Sort of a "default" program nowadays. There weren't many options available at the time on simple systems like these.

Ed Vaughn, MBSWWYPBX