Possibility 1: Ariens did not know that Mobilco was initially expunging their logos and only using their own (perhaps because the Ariens decal was taking up the very real estate where Mobilco wanted to put their own decal). Mobilco may only have done that for a short time, then stopped.
Possibility 2: If I recall correctly, there was no chassis ID plate on your mowers when you obtained them, but was there a plate earlier? In other words, are there pop-rivet holes in the vertical web of the frame rails, or wherever Ariens puts their ID plate? If the mowers originally had the Ariens ID plate, but no logos, that would meet the Ariens requirement that their brand not be expunged "completely".
Possibility 3: There was an Ariens decal on the machines when they were sold new by Mobilco, but it fell off or was removed subsequently.
Essentially, I doubt that Ariens had much knowledge, or control, of what their machines looked like by the time Mobilco sold them back in 1960.
That guy in your picture looks like he is setting out to mow the entire Nullabor Plain with a 20" mower. Lucky he's young, he could be out there a while.
