Standard Modular System
The Standard Modular System (SMS) was a circuit packaging system developed by IBM. Developed in early 1958, it was initially used in Stretch, and in machines of the IBM 7000 series (the first use was in the IBM 7090, first shipped in December, 1959).
Although IBM had developed standard 8-tube modules for the IBM 700 series, due to lax control over the creation of new types, there were eventually over two thousand different types of these modules used in the 700 series. There was also a desire for automated production of modules, for cost reasons; machinery was developed to insert components in PCBs.
After some internal debate. SMS was proposed, with cards of 2-1/2" by 4-1/2", with gold-plated contact fingers on one side of the card, which was to be inserted in a backplane, which used spring-loaded phosphor-bronze contacts. (So called 'two-wide' cards, double the width, were used in some machines, such as Stretch.) Wire-wrap was used to interconnect the pins in the backplane.
SMS was used in all IBM computers until the Solid Logic Technology system used in the IBM System/360 was developed in 1964; but even then, SMS continued to be very widely used in less demanding applications, such as peripherals.