CAPS-11 is the Casette Programming System for the PDP-11. The manual is available at Bitsavers, binaries can be found at SimH.
The manual contains instructions how to assemble the system from source, the source code should be available somewhere.
I have disassembled the code, but annotated source would be really useful. I have googled around and not found anything.
(I am 100% pdp11 novice writing pdp11 emulator from scratch for kicks)