They just ran the Fuel Feed Rubber Hose (on Incoming Line from the Fuel Tank)
Directly to the Lift Pump...Bypassing the "Fuel Thermostat" (Heater).

Howeveah, your Primary Fuel filter should still be InLine between the Tank Line
and the Lift Pump.

There's an O-Ring inside the front of the Fuel Thermostat,secured by a "C" clip
that starts seeping Diesel after years.
(No big deal to replace,unless you're a shop without the O-ring
[Or the Knowledge where to get one]...)

__________________
'84 300SD sold
124.128