[D.S.]

The pump has a recess for an O-ring to fit between it and the reservior then a bolt goes in to hold the pump to the res and complete the seal...

You can see in your pic in post #4 a bolt with a stud to attach it to a bracket, if there is no bracket on the rear you use a plain bolt but they have a larger than normal head to seal better...

 

[D.S.]

It's a square O ring between the reservoir and pump, if it was purchased as a unit it's probably in there and just needs a bolt to seal it up...