The hole in the water passage is for a mechanical temp gauge sender. Pretty common, but there isn't a lot of material there so be careful if you plan to use it or plug it with a pipe plug.

"Follow me the wise man said, but he walked behind"


'92 D250 Club Cab CTD, 47RH conversion, pump tweaks, injectors, rear disc and hydroboost conversion.
'74 W200 Crew Cab 360, NV4500, D44, D60 and NP205 divorced transfer case. Rear disc and hydroboost conversion.
2019 1500 Long Horn Crew Cab 4WD, 5.7 Hemi.