What you have there is the business end of Lawn Boy Loafer (riding mower). Do you have the rider powered unit 9213 that fastens behind it?
I am very sure that it has the mag deck and <MAGNESIUM> should be cast in smaller type in the casting on top of the deck just above the larger type PLACE FOOT HERE WHEN STARTING. At least that it is that way on mag decks of my 1962 - 7252, 1964-7253, 1969 - 5238, 1973- 3058 and 1974 -5023. Look closely it's not very large type.
I just got new NOS short block in the box a couple of weeks ago, but I'm going to hang on to it. There has been one on ebay for $199.00 but I didn't want one that bad. Let me know if you see that <MAGNESIUM> mark on yours.