Long time ago I have bought three MX300.
Two where Intel based and one where a NSC based System.
One of the Intel based Systems and the huge amount of documentation which was mostly Sinix 5.2x was lost over the years.
Here are some Pics from the MX300-15 (NSC Based)
My extensive search in the basement was successful, as I found the Sinix Installation Set which I had bought many many years before.
Sinix 5.21 with Authorization Floppy, maybe the last set worldwide.