OK thank you all for the insights.

I'll go ahead and remove the hose etc. and look for the 'bleed screw' also.
It was just that the reservoir hose seemed small diameter for an intuitive radiator filling purpose...

I'll take the opportunity to perhaps fit in an aluminum fitting to add strength to the plastic neck.
Yup, my replacement Behr radiator years ago still came with a plastic neck !

__________________
1979 300D 220 K miles
1995 C280 109 K miles
1992 Cadillac Eldorado Touring Coupe 57K miles SOLD
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1979 240D 140Kmiles (bought for parents) *SOLD.
SAN FRANCISCO/(*San Diego)
1989 300SE 148 K miles *SOLD