856 guzzler or sipper?

I have a question however, that I've not seen addressed in my reading so far, so I thought I'd tap into that knowledge base in an effort to find the answer.

I'm a wet behind the ears tractor fan, my sole experience to this point amounting to a couple of years with a Ford 800 doing chore-type work around the homestead a number of years ago. Fast forward to the present, and I find myself very much interested in acquiring a 1969 856 with a diesel between the rails that I've come upon. The serial plate lists the model as F 856 sans the "D" that I've seen on other diesel equipped machines. My question: Is it an airtight rule that the serial plate would always denote a factory diesel with a "D" on the plate, or was it a "fast and loose" rule that allowed for some variance? Obviously, what I'm wondering is if this beast was born as a gas guzzler and at some point was reborn as an oil sipper. Any light that can be shed will be greatly appreciated. Thanks gents.

Further research here and elsewhere though has so far proven fruitless in deciphering the data of the "KIND" or "CODE" portion of the tag, which reads as "2610142" followed of course by "U" then the actual serial. Do you know the info to crack the kind and code mystery? You've answered my original question concerning the lack of the "D" which I much appreciate, now if my curiosity about the kind and code data can be satisfied it'll be icing on the cake. I surmise that such info can be found in certain books and texts relating to all things IH/Farmall, but being a neophyte I've no data library to refer to. Thanks much.

Having said that, I'm still in the dark about this serial plate conundrum.

I know by the time 66 series came along in 72, only the 766 and 666 were being offered as gas. Bear in mind my buyers guides are Canadian. IH printed different guides for US and Canada and indeed offered different options in the two countries.

So, the plate data format change occurred some time mid year of the '69 model run. A trivial fact in the bigger scheme of things, but one that nonetheless is interesting from the standpoint of change in the history of the manufacturer. Someone, somewhere, owns a '69 IH/Farmall that was the first to wear the new format plate. Again, trivial, but somewhat interesting to ponder where that machine ended up and what duties it performed.