The biggest challenge here is the lack of a manual or documentation, which is understandable since FORTH was a language with a single person using it - Chuck Moore - back in the 1968-70 timeframe. Coupled with that lack, the language evolved quite a bit before the versions known to the public and covered in available material.
As an example, in more modern FORTH implementations, the verb . (period) will take the top element off the stack and print it. In this implementation, the period is a synonym for : (colon) and starts a definition.
There must be a command to pop the top element and print it, but I can't figure it out. I have been trying every verb or verb sequence I can think of. So far, it has defeated me. There are only a couple hundred entries in the dictionary but when you include combinations then a random walk approach is going to consume a lot of time.
The other major issue is understanding how to use the disk block and editing features. The verbs like CREATE, DELETE, ACTIVATE and INTERPRET will act on disk files that are managed by this FORTH code, but so far I haven't pieced together enough to do what I want, which is to create a file, store some FORTH statements in the file then cause them to be executed.
These are the forerunners of the 1024 word disk blocks and verbs in more modern FORTH but far enough away that the modern verbs and procedures don't work with the original FORTH.
The expansion from the few primitives up to the running FORTH system was produced by a bit over 200 lines of FORTH statements but these are an admixture of 1130 machine instructions and text strings, not simply a series of FORTH phrases as you would see in a more current implementation.
Thus, understanding the code takes quite a bit of effort, leaping back and forth between the 1130 hardware and the FORTH verbs and nouns. It is slow going.