The Acorn System Computers were housed in Card Frames manufactured by Vero, the original Vero Company has changed owners and name over the years, but is still trading now as VeroTec (verotec.co.uk).
There seem to be few original System Computers in existence and even less photos on the web so it a takes a bit of detective work to determine the details of the Card Frames, fortunately I have an original System 2 Card Frame and there are some details (part numbers) in the System 2 and 5 manuals.
The System 2 Computers are housed in Vero KM6 Card Frames, the System 2 uses the type with 'formed' End Plates with fixed positions for the Tie Bars. I assume that as the System 3 was a similar size (3U high) this was the same. The System 4 was a double height card frame and photo's (Chris's Acorns) show that it was simply two 3U Card Frames mounted on top of each other. The System 5 was a departure from the earlier computers being housed in a Vero KM6 Card Frame with 'flat' End Plates. The 'flat' End Plates provide more flexibility for Tie Bar positions but required the use of plastic Location Mouldings between the End Plates and Tie Bars.
Vero KM6 Card Frames are no longer available new though they do occasionally pop-up on eBay. Fortunately VeroTec sell the KM6-II which is an 'improved' and compatible version of the original Vero KM6 (the plastic Location Mouldings are no longer required).
The following pages detail as far as I can the build for each of the four System Computer Card Frames and include compatible VeroTec KM6-II part numbers enabling Card Frames to be purchased and built from new parts.
Thanks to a company in New Zealand, www.casa.co.nz, I have copies of some sections of an early Vero Catalogue which are useful references, they were kind enough to let me place copies here,
Here is a copy of their stock list of Vero parts:
Unfortunately postage costs make it rather prohibitive to UK buyers ! :