Your 261 housing outlet is different from both of my 261 outlets (1959 & 1960).
I'd guess the 235 outlet (radiator hose fitting) has a 1.5" outer diameter, and both my 261 outlets (radiator hose fitting) have a 1.75" outer diameter.
Furthermore: My 235 "cover" will bolt to the 261 base, but it cannot hold/enclose the 261 thermostat. The 235 base and 235 cover cannot hold a 261 thermostat.
The diameter of the 261 thermostat is wider than the diameter of the 235 thermostat.
If I have time tomorrow, I'll post some photos.
Your 261 thermostat housing is distinctly different from my two 261 thermostat housings.
