The manual is available from Pentax at
http://www.pentaximaging.com. Click on "Support" and follow the links. They have manuals for all Pentax cameras from about 1960 on.
In case you weren't aware of this, the Spotmatic IIa was a US-only variant. There is no Asahi version of the IIa. It was made to work with the Rollei-made line of Strobonar flashes, imported at that time, by Honeywell.
The sensor on the front of your camera did the work of the one on the flash. In this way, the light-sensing was on the camera, rather than the flash, which is better, if the flash is off-camera. After all, its the light reaching the camera that matters.
I believe the Strobonar 772 and 882 were the models that worked with the Spotmatic IIa.
I have never read any explanation of how the camera-to-flash communication took place. AFAIK, the setup uses a standard PC-to-Household synch cable, just like all the "potato masher" style Strobonars. There are no additional cables to carry the light sensor information back to the flash.
If this is your first Spotmatic, you're in for a treat. They are terrific cameras. I have two and I still find them to be the most ergonomic and fun-to-hold cameras that I have encountered.