MFJ was at one time said to stand for "Mighty Fine Junk".
I suspect your dummy-load may have been overcooked at some time.
At junk-sales/hamfests/rallies you come across Heath "Cantenna" dummy-loads - oil-cooled beasties built into what looks like a paint-can. These were designed for the US market where Kilowatt-capable ham radio stations were commonplace - so Cantennas can take quite a thrashing.
https://www.eham.net/reviews/detail/13123
My dummy-load-of-choice when I was designing juicy RF-stuff was a big grey-finned beastie about the size of a large load-of-bread, by "Aspen Electronics" - you occasionally see these at rallies etc and they still sell for 3-figure sums.