I found this video about the class 51 machine. I think it is a flatbed version of the class 21. I guess, that it have got the same shuttle hook system as the class 21 and perhaps the later series 1000 and 2000 machines - do you know that?
But this video states in text, that the machine is able to sew Tex 90 thread without issue.
https://youtu.be/2kAp8OxHjV8