Very tenuous guess here for Oshiyama, take it with a grain of salt.

I'd wager it's either Oshiyama or Oka, as it just doesn't remind me of Ishihara's idiosyncratic output during this era, and i can't imagine anyone else pulling this off.

Oka was very strong around this time, and the timing might be just right for Oshiyama to attempt something like this.