Strange third party board in PDP-11/45

[Mattis Lind]

This board was sitting in slot 21 of the backplane in a 11/45

https://i.imgur.com/ZYWZQCo.jpg

What kind of board is this?

It has 26 bipolar RAMS. Fairchild 93415 1kbit SRAM.

The manufacturer might be ACT whatever that is.

My guess is that it is some kind of cache board? It is connected to both
unibuses in the machine.

Better ideas? Documentation?

/Mattis