MGT was an early SGF[1] program. The MGT program was originally developed to be a companion for the series, "From My Go Teacher" which is a collection of problems and study material that can still be found on the net.
It used ASCII graphics for displaying the board not much unlike the syntax SL is using today.
Related links:
[1] Note: the file format was not called SGF back then, but was just the file format of Smart Game Board?, a Mac application developed among others by Martin Mueller and Anders Kierulf.