Help sorting out mech. fuel pump?

[cam.man67]

Decided to replace the fuel pump on Lester, we'll see if it gets him running again. Got fuel up to there, so it should. Here's the confusing part:

The pump I pulled off and the new one look identical...except that teh old one had a line close tot he inlet. Oddly it ran to the fuel filter. It doesn't look like the primary fuel inlet, so my question is what is the purpose that inlet/oulet/bypass/whatever? Do I need it? Is it a deal-break that the new pump doesn't have it? Does it have something to do with the fact that the orignial engine was a Olds diesel? The old pump part number is Carter 0-1955.