The .MAS extension is because it contains a bunch of files together. A program on the DEC-10 called SUBFIL was used to collect files and create one big file with the sub-files in it; the same program could split them apart. It's a bit like a .ZIP file, except without the encryption (because if it were encrypted, you couldn't print it out). As for why we wanted to package files like this, well the main reason was because each individual file came with an overhead of 3 blocks (each block being half a kilobyte of 36-bit words). If you had lots of files, those blocks added up and you could easily exceed your storage limit. We therefore preferred to keep our files in .MAS format rather than as lots of smaller files. To this day, I still don't like creating programs that have separate files for each individual class or function or whatever.
Oh, the reason the job name is THIRD is because I printed out the MUD code itself and the MUDLIB library that interfaced it to the operating system. These were FIRST and SECOND. If I'd called the jobs MUD, the operators would have spotted what I was printing and told me off for wasting paper...