Hi Tony
Just went out to the shop and looked in the doors of the HUP through the lock assembly hole. Both doors have full length plates which is much heavier gage steel than the door shell.
One of mine my backing plates was loose enough that when drilling the holes it started pushing the backing plate away, so stuck the hinge in place with the bolts to hold the layers tight together while drilling and welding.
Fix the front hinge area on my HUP for the first time some 25 years ago, so if the 2nd repair last as long I won't complain much.
The loading on the hinges of the front door of the Pattern 13 are ridiculous because they are placed so close together. Interestingly the side and rear doors of the HUP have a hole at the bottom of the door that would allow you to slide a new or additional plate in. Not sure what that would do to the hinge alignment though.
In the picture below you can see cracks in mine drivers side door, you are looking at the lower hinge are, if you look at the where the outer skin is folded over you can just see a little line of rust which is the area of movement between the outer skin and the inner door panel.
I'll try to remember to gather all of the door hinge before and after photos an post them on my web site.
Hope this is of help to you and others at this point in their Pattern 13 restorations or ongoing maintenance.
Cheers Phil