I use a number of forums, and I've not seen anything like this one before. I suspect that it is NOT a pre-build package, but may well be a one-off system developed specifically for TMA by a supporter of the organisation. I've had a look at some of the source screens, and there's nothing there to hint at anything else.
There are certainly some peculiarities here. Cannot explain some of them. One particular example is the ability to create a 'locked' thread, which requires a password to even read the thread. It appears that it is easy to create such a thread, maybe even accidently. Not even sure that the original creator can then do anything with it?
I've not seen any sign of any instructions/manual, so you're on your own there. However, all the basic things seem to be obvious enough, and I at least seem to have been able to manage OK.
Not aware of any preview, but if you just save your posting, you can immed view the posted item, and you then have the ability to edit it (or even delete totally).