This little pcb was created by Radich Electronics back in the day to convert a bootleg defender boardset into a Robotron boardset.
But where is the Special chip 1 pair you say? All the TTL logic above the roms implement the blitter chips. In a attempt to prevent copying they have sanded off all the identifying labels.. even underneath the chips!
Fortunately nowadays we have chip testers that can identify any TTL chips, so I am currently labourously desoldering and identifying each chip. Next step is to scan the pcb and make a net list to create a schematic. There are tools out here to assist with reverse engineering but they are all expensive, or require paying someone else to do the work.