I am having trouble obtaining this seal. All parts stores I've contacted so far only have the seal for the 3/4" shaft which to my understanding is '79 & later. What I have is a '76 with a 7/8" shaft. Seals are 1 3/8" O.D. What I've found so far is also smaller on the O.D. Please educate / advise me on what I am missing. Where is this seal available? Or is it?

if you can get numbers from the seal, if it's still made by anyone, you should be still able to obtain one from somewhere. [like a bearing house]
i'm thinking this application of seal is possibly used on applications other than steering boxes.
just my personal experience with manufacturing and farming equipment.

Yup. I have to "match up" seals all the time especially on the garden tractors and such that I work on...
I was surprised when I bought my 2 post lift there's 14 ball bearings on it... Going by the numbers on them they are the same bearing as a input shaft bearing on a Ford 4 speed Top loader trans... If you have the shaft Diameter and the recess diameter where the outside of the seal rides it can be cross referenced easily

Go to a bearing supplier. They will measure the old seal (if you have it) and sell you a new one.

Unfortunately, not that simple. There is an oil seal, a dust seal, and O-ring. I do have the old seals but searching keeps returning links to parts stores and showing the wrong kit (3/4") . None of the web pages show ID & OD. I have found the old Gates number. Gates P/S Repair Kit - 351310 It seems as only Jegs and Ebay have them available at highway robbery prices. Although I did find reasonable price on Ebay, but only available apparently if I create an ebay account.

Jegs Listing

I bought a rebuild kit from NAPA years back. Did you try them?

A bearing supplier like Purvis bearing will have all 3 items.

I have tried Napa. Just like the others they show the 3/4" seal as a fit and is the only one they carry or can order. Had never heard of Purvis, and got excited when I saw a branch here Denver which is reasonable drive time. The gentlemen I spoke with indicated he will see what he can find, but it didn't sound very promising. Thank you for the tip.

what number[s] is [are] on the old seal[s] ?
o-rings can be measured easily, and are readily obtained almost everywhere.

You are not looking very hard. Seal kit

and it even fits fourty'leven different applications...............

Original seals are IPC 5686526 for the oil seal & IPC 5688229 for the dust seal. You will find them on the web in multiple places all showing discontinued. I did find online the oil seal N.O.S. at a surplus reseller, but no dust seal.

Nitrousin, I had found that and in fact it is in my cart. Rock Auto also carries the same brand & part number. I have not pulled the trigger for this reason. Nothing I've found verifies it as for the 7/8" shaft. I've already been through that long list of applications that it "fits" with Napa, O'Reilly, Advance Auto, & Autozone. They all say they have it and it fits. But when you get hands on it, it is the seal for 3/4" input. The Gates kit I listed before is so far the only verified correct items.

Thank you for the time you spent looking and responding.

Excuse me for asking but is your truck a 2wd, 4wd, 1/2ton, 3/4 or 1 ton?

2wd 1/2 ton short bed mongrel. I picked it up this way a couple of years ago. It has been put together from more than one. Got two vin plates with it and it had a state issued vin. Because of this, I tried looking up various years, 1/2 ton, 1/4, all ranges of engine sizes, D100 / W100, Amazingly they all come back with the same part number for the seal. As do numerous early and mid 70's chevy cars and pickups. I found one of the Gates kits on a web site catering to 70's camaro's. Don't know if it supply chain issues from the covid mess, or just one of those quantity moving items that is limited in production.

I used Edelmann Seal Kit 7860 for my 76 D100 2wd, It took afew incorrect kits to find one that actually fit mine.

Thank you. I see you have been through the same struggle. That is confirmation enough for me to order one.

Thought I would provide an update. The Edelmann 7860 kit arrived. I measured the input seals and confirmed they are for the larger 7/8" shaft.

Thanks Azzkikr for that number.