missing expansion tank

I have a Wells-McLain Company (Michigan City, Indiana) Size 5-W-19, Series D. oil boiler that was converted to gas....it is forced hot water with radiators...ONE floor in a bungelow style home built 1928.
The problem is there is NO expansion tank to be found.....the Gas company put an overflow pipe next to the boiler a few years ago (none was there before) and now it drips a gallon and a half a day......any help appreciated as to what I should do here....(I pay for my water use).

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