Sending units

I have two new oil pressure sending units that came with the project
that I purchased. They look identical other than one has a single post
on top W/nut and the other has two posts. There is no instructions in
the boxes and the VDO gage is already installed . Is one better that
the other? Any suggestions as to which one should I use and sell the
other?

sounds like one is the switch for an idiot lite, with closure to grd
and the other a resistance unit for a gauge or the two terminal could
also be a switch but isolated from grd. this would be used to start
the hr meter once oil pressure is established. if u had some way to
pressure check them and measure the terminals resistance to case of to
each others, it would answer the question. otherwise, see if u can
google the p/n's or maybe take them by an autoparts store see if then
can identify the function of each by the p/n's.