Have a look at dieselinjectionpumps.co.uk Parts and information/diagrams available.
I think that cover is for the pump advance mechanism, a spring/plunger is in there. Cover has a gasket or O ring to seal.
I am no expert on diesel pumps but parts/info from this firm has helped me repair both...
Your Landy is not a Defender [ the name came later on] Yours is a Land Rover One Ten, this detail can make a difference when ordering parts. Looks like a Lucas 127 type alternator.
Looking pretty good to me. What engine? From the TDI decal on the wing a 200tdi? Common enough change for early Ninety [written not numbers, can make a difference when looking for parts] Mine is a 1990 Ninety with 200tdi.
I would put a spanner on that union with the rubber pipe and see if it will nip up a tad. As you say oil may be coming from above but that is the diesel injector pump. Air flow when driving can sling oil all over.
When Land Rover brought out the Series 3 they started the suffix at A again. But now just to cause us confusion the location is on the top edge of the transfer box.
Then there is the issue of what other folk have done in the past with fitting parts and cases from different eras.
One of mine went to leak recently, no issue with the ball housing but found a bit of water had got into the back of the seal and rotted the seal spring until it gave way.
That is the way I do it if other parts in good order. When out I lower the hub swivel unit back into the wheel laid flat on the ground, holds it nice for seal change. Extra hands to get them back is good, more so the long shaft.
It doe's sound like there is a lot of "slop" in the system. Can you move arm by hand to full sweep and usual park position?
If so there can be issues with location of arm on its spindle or with the drive cable itself. Also can be problem with location of drive cable with the motor gear box.
What model is your Land Rover? There are several different Series box and each has a different Suffix number/letter. Gears look different to me and so will not do. You could swap the whole transfer box section.
Moghrey mie and Bore da, From the West Wales coast. I listen to Manx radio a fair bit as it comes in well hear and has good music. Try carpricetracker.net May help with historical prices. Hwyl.
In many years gone by I did use the 36IV for other uses other than my Land Rover.
I had one on my 1300 Escort.
Experiments with different jets had me a good cheap power up grade [along with other tuning] over the original set up.
Probably cold rolled, likely the most available. Strength wise not much in it for what you need, but cold has a smoother surface finish. 3mm is what I use for chassis.
The only thing I have done to sort a crunched wing id replace the whole thing with a better [not new] one.
However recon you are on more of a restoration than fix up. It is not hard to remove spot welds, there are special drill bits to do just that and they work well. It is a case of rivet to...