I have a couple of these. One is a very very ratty one which was the second pdp I ever had - I picked it up from DEC at Basingstoke back in around 1991. Wasn't in good shape when I got it, is in worse shape now - there was quite a lot of corrosion in it. I have spares tho, including a *new* backplane and power supply, so it's restorable.
The other machine is in very nice order, was collected from British Aerospace at Preston along with a bunch of pdp-8e machines, and a pdp-15.
In good shape, in a rack with RK05 & RX01/2.
The 8a is... not my favourite machine. Has a similar uninspiring front panel to the 11/04 & 11/34. Last Omnibus pdp-8. Came in two sizes - a 10" full-depth chassis with the PSU in the back and a 'full-height' Omnibus backplane, and a 'lite' version which was half the depth, with the PSU in the bottom of the chassis and a 'half-height' Omnibus in the top. The 'programmers console' was optional: some systems were configured with just the lower panel, with power/run/halt switches only.
Some used the same CPU card set as the 8/e etc., others used a single-board CPU (KK8A), which was hex-width - the 8a can take certain hex-width Omnibus cards, such as the RL disk controller - as well as standard quad-width Omnibus cards. Could use core or MOS memory. (The cards could be hex-width, but the Omnibus itself was still quad-width!).
http://www.faqs.org/faqs/dec-faq/pdp8-models/section-11.html There's a useful FAQ...
http://www.xs4all.nl/~andreth/pdp8a.htm This chap in the Netherlands has some good pics
http://www.arcula.demon.co.uk/eight4.htm Excellent page, showing the different between the 'large' and 'small' chassis 8a.