Brother KH521 knitting machine is a 4-push button machine with 200 needles, spaced 4.5 apart.
It was manufactured in the early 1960s. It is possible that it was released for the Japanese market only judging by the number of writing on the packaging, on the machine and on the carriage in Japanese.
I personally did not have a chance to work on this machine but it is very similar to other 4-push button machines, like Brother KH531 and KH551. Thus, it should operate similar and have the same pros and cons.
Because of its similarity with KH551, Brother KH521 might be compatible with the Brother KR551 ribber.
Below are the pictures from the Internet published with permission.