deskthority

I was talking to a friend about the differences between the two main Spanish-language layouts and he asked me something I didn't really know and hadn't really thought about: where are the formal definitions and where did they come from?

In some cases, tracing the origins of a particular national layout isn't too difficult (for example, the Brazilian ABNT keyboard comes from... the Brazilian ABNT organization), but some others simply escape me. The Spanish (Latin America) layout, in particular... I haven't got the slightest clue as to where it came from, and googling it yields no usable results.

ISO itself? A particular formal standard in, say, Mexico, that later got applied to "everywhere south of the Río Grande River"? Someone in IBM a few decades ago, out of a particular scratch he itched? Copied from the layout that some type-writer company made for the Spanish-writing market? I just don't know, and I really want to figure this one out...

I don't think there is one governing body for all layouts. There are standards organisations in many countries ... and then there are specific vendors who adopt one layout or the other, making it the de-facto standard - which is then adopted by other vendors because of familiarity.
I think that in many national standards organisations in Europe are more aligned with the ISO standards in general, where as USA more often ignores International standards for their own.

There are still minor differences between Apple Mac and MS Windows PC layouts in many regions. The symbols on the keys to the left of 1 and left of Z are different for UK. Left of 1 is different for Swedish/Finnish. The location of @ is different for German (Alt Gr+L vs Alt Gr+Q).

The X windowing system, used for Linux and Unix variants appears to have adopted the layouts from MS Windows because those operating systems are often run on PCs anyway.

I wrote about the history of the Return/Enter key in the Wiki. It used to be 2×2 units on typewriters.. and then more keys for more symbols came along, moved the Return key right and bit off chunks from it.
A lot of this happened on revisions of the Selectric line: Selectric II and III, for different markets. I have a vague memory of a story of one such change getting a lot of criticism from US customers, which led to the key being changed back only for the US market - but I have not been able to find any source of that.
I believe a large reason for the narrow vertical and narrow horizontal keys on computers are because they are easier to make stable if they are smaller. Typewriters had them stable because they were on large hefty arms but many vintage computer keyboards from the '70s and '80s did not even have stabilisers under the Enter key.

One keyboard that was very influential was the DEC LK-201.
The IBM Selectric set many of the de-facto standards for a time and I think IBM copied the Selectric's different layouts when it designed the Model F and then the Model M.

The ASCII table was set up so that shifted symbols on the numeric row would be different with only one bit - but it followed an older convention for mechanical typewriters that was more common in Europe than in the US at the time. There were then keyboards that produced ASCII that had that layout.