It's the head seal. If you opt to replace this seal with the pump installed in the car using Dieselgeek's procedure, I urge you to rotate the engine until the pump is in the middle of an injection stroke. By doing so, you increase the spring tension holding the internal pieces of the pump together and minimize the possibility of dropping rollers, thrust washers, or the plunger shim into your pump and breaking it.
To find when the pump is in the middle of an injection:
Remove the timing plug located in the hydraulic head centered between the 4 deliver valves (where the lines attach) and remove it. It's a 12mm plug.
Remove the plug and find a drill bit slightly smaller than the pin at the end of it.
Next insert the drill bit into the end of the head.
Rotate the engine (better yet, have an assistant do it) and notice how the drill bit is driven out by the end of the plunger.
When the plunger is at it's maximum lift, (~4mm) the return springs are at maximum tension inside the pump. This is the safest time to try to back the head of the pump out and replace the seal.
Good luck.