This data was provided by John Shriver on March 22, 2021. He wrote:

I have a PDP-1X Microtape (a/k/a DECtape) which I have had for years which I finally got read and managed to decipher, that I’d like to share with the SIMH and PDP-1 community, probably by having it stored on Bitsavers.

It originally was David Thiel’s tape. (MIT BS 70, MS 72.) I have his permission to share it. This tape was living with the PDP-1X when it was at the MIT Electronics Research Society in Building 20, sharing a 10x15 foot air-conditioned room with their PDP-7. When it was leaving for the Computer Museum at DEC Marlborough, Dave Felsenthal gave me this tape, saying that it had Spacewar source code on it.

With Dave Gesswein’s help, I got the bits off the tape. With a bunch of detective work, I cracked the interleave, block map, and directory structure of the tape, and managed to get all the files out in coherent form.

It indeed has Spacewar! source code on it. One version is for PDP-1. All the others are newer versions that use the new instructions added to the PDP-1X, with their upper-case opcodes in assembly language. A snippet of the newest version:

[...]

It also has the binary of Microtape File System in its first 13 logical blocks. That’s a piece of the PDP-1X software that I don’t see any source code listings of online.

There’s also a good bit of other PDP-1X assembly language on this tape, including a floating point package. It probably would be very useful if anyone was ambitious enough to try and add a PDP-1X simulator to SIMH, providing known working code.