When I lost track of the heater valve orientation while rebuilding the heater core in my 110, I set it by blowing through the valve until it was obvious where it was full flow, then mounting the valve arm at full extension of the cable.

Or you can try setting your heater cable to full heat (that would be full extension and referencing the photo above) and insert the valve cartridge so that its hole fully aligns with the outlet in the valve body. That should allow for the square opening in the valve arm to fit onto the square fitting in the valve cartridge properly.

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1981 300TD 310k miles
1970 280sel 172k miles
1966 230 Fintail 162k miles

"Where are we going? And why am I in this hand basket?"