Acorn's first computer was the Acorn Microcomputer, designed in 1978 by Sophie Wilson and shipped in April 1979. The System computers are all based on Eurocards, the Acorn Microcomputer has 2 - 1 CPU and 1 x keyboard, the other models are rack mounted with a selection of Eurocards. This means it is not easy to define a "standard" System computer.
For a description of the System computers see F J Kraan's The Acorn Atom pre-history . In particular see the Control Universal catalog which includes the Acorn System 1 - 4A and many of the Acorn Eurocards used in them. I think the catalogue dates from 1980-81 because it does not include the System 5. I have used the descriptions of the computers and eurocards extensively in these pages.
Here is a catalogue of System Computers and Eurocards. It is incomplete, but it describes many of the Acorn Eurocards.
Details of the Acorn eurocards are available in 8-bit upgrades under Atom and System Upgrades.