In support of my two Compaq Portables (II and III), I got myself a copy of the official Compaq 3.1 version of MS-DOS and BASIC. At least for the Compaq Portable II, my understanding is that this is the latest version (oddly, apart from MS-DOS 6.0) that it can run. I had a version of these disks already from the net, but those were in German—these disks are not. I also scanned in the manuals and the box. Links to the disk images (360k .img files) and the manuals are below. I’ve successfully booted from them and briefly used them from within VirtualBox, so they should be ok. Next up will be to create real floppies from the images and install the system on the Compaq Portables, but one thing at a time.
Links to the scans: