Was looking to verify the D100 I had bought did in fact have a 318 and the first thing I noticed was the iH. I will try to get a picture and share this evening. Just interesting...
I've run across a few, but I wouldn't call them common... IH was just one of a few different subcontractors that did foundry runs for Chrysler. The rumor mill says they're a bit heavier of a casting, but I never weighed one or noticed anything visually different other than the foundry marks.
I had one that came from a 77 Lebaron.
Remember if you had a international pickup from the 70s and it had an automatic transmission in it, that trans was a 727.