I’m afraid I can’t make sense of this paragraph:

Did you accidentally omit some words and/or include some additional ones?

No.

The linked-to web page and the video in the post after your post (which I had not seen until after I had posted the post that follows your post) has sheets of machine made deckle edge paper with four purported deckle edges, two real deckle edges from the process, two faux deckle edges from cutting.

Yet my experience (then as I wrote)

A ream of it is 500 sheets.

Large Post is a size of paper, like A3 and A4 are sizes of paper.

The term paper size is ambiguous as it may mean size of paper as above, or a type of substance used in papermaking.

So the ream of deckle edge paper that I saw and used in the 1960s was cut mechanically with a straight edge.

As the sheets were cut by me using a Dryad card cutter, some of the thus produced smaller sheets had one deckle edge, and some had no deckle edge.

These sheets were for letterpress printing upon, using an Adana 8 x 5 printing press, a hand operated purely mechanical printing press.

Does that help? I realize that you had posted before I posted the link to the video where people can be seen hand cutting the paper to produce the faux deckle edges.

Well, some readers might not know that a ream is a quantity of paper, and that that quantity is 500 sheets regardless of the size  of the sheet, though probably each sheet nominally the same size as every other sheet in the ream. Someone might wonder if it is some vague term like "a lorry load" or if thought to be a specific quantity, wonder if it is some special purpose measure of volume or weight, like when articles have some unit such as "the size of Wales" or "the population of Skegness" in a phrase such a "The fictional town of Roseville, Kansas, has a population the same as the population of Skegness."

Well, I now realize that the word 'was' is duplicated. That is an error.

I will rewrite it.

My experience of machine made deckle edge paper is that a ream of it was purchased. Each sheet was Large Post size. On each sheet, two of the edges, which were opposite each other, were deckled edged. The other two edges were straight cut, not deckled, thus neither real deckle edgednor faux deckle edged.

I had an aunt in the paper trade, and to her a ream was 480 sheets. Apparently it has now become 500 sheets, having grown toward the end of last century. I blame the EU!

Whilst "reams" can be a vague term of quantity, eg "William has created reams of threads in this forum", there is nothing vague about a "ream of paper". It is internationally accepted nowadays as 500 sheets precisely.

I don't know the population of Skegness.

Looking up "ream" in my Chambers dictionary, I see there is also a "printer's ream", which is 516 sheets. Would this be for a reason much the same as 13 being a "baker's dozen"?

https://www.sizes.com/units/ream.htm provides this information:

Chambers also states that in your part of the world, "ream" also means cream or froth, derived from Old English, as opposed to the usual meaning, which is derived from Arabic. Is ream still used that way in Scotland?