A stock (to form the clear jelly) is always added to the hand-cut pork pie filling after baking, it is added because the meat expands during cooking and shrinks on cooling, leaving a gap under the pie lid.
The addition of the jelly after baking fills this airspace created, which helps preserve the pie and keep it moist. A properly made jelly from a traditional Melton Mowbray pork stock (see the next page in this recipe) also adds the ‘secret-sauce’ to this seemingly very simple pie filling. Added in warm (and still liquid) the stock flows over (covering and coating) the cooked meat in the baked pie, before setting into a subtle tasting clear jelly; and together, this is what makes this pie world-famous.
http://historicalfood...wbray-pork-pie-recipe
there is a pork pie appreciation society as well