This sucks. I Googled the origin of the Pause symbol, which led me to Wikipedia, and it had a definitive answer. Wanting to look at the cite for that information, the citation was this thread here on SDMB. 
lol thats how i found this thread. it was also linked in some article about the play/pause/stop symbols. u guys are a primary source 
That’s usually not a great thing. 
I think one critical question is to consider what the pause button does. On videotape machines it pauses the tape and leaves the machine displaying a single frame. But this is something of a difficult trick. It requires that a useful amount of a frame can be recovered by the spinning head whilst the tape is still.
The 2 inch “Quad” format video recorders used either 16 (for NTSC) or 20 (for PAL) stripes per frame. If the tape stopped the heads scanned only that fraction of a frame. Thus they could not usefully pause, and they had no pause button. Indeed they provided little more than record and play. Editing was done by splicing tape - since the stripes ran 90 degrees to the tape movement it was possible to make spices - unlike helical scan formats. These machines were the dominant professional VTR until type B and C machines took over in the mid to late 70’s.
Ampex designed the type A VTR machines with 1" tape and a helical scan. But they couldn’t do broadcast video and didn’t catch on. They could however freeze frame. The instructions just told the operator to hit the stop button (which would stop the tape advance, but keep the head rotating) and a still picture would be presented. If the heads were across two frames the operator needed to manually move the tape (by turning the reels slightly) until a stable image resulted. There was thus no pause button.
In 1976 the 1 inch format aka type “C” with helical scan was introduced, and it could also recover enough of a frame to display, and could perform freeze frame, jog, and slow motion. This was the first widely used VTR that could pause. This format was a cooperative development by Ampex and Sony. (Type B was developed in Europe at the same time and is largely the same format.)
The 1976 Hitachi professional portable VTR for type C has the double bar pause symbol. The Ampex and Sony of the same year don’t. The Sony had a hybrid bar and triangle that denoted access to the various jog functions. Maybe it was inspired by the double bar.
But oddly, in 1976 Ampex released an audio tape machine - the ATR-700 - and it used a different symbol for pause. A blue dot with a horizontal white line with a v shaped dip in the middle. Quite a nice pause symbol as well, but not the double bar. So one wonders what Ampex were doing here. If they invented the double bar symbol it is odd they didn’t use it here.
All the later VTR formats used helical scan as well, and they could pause as well. Which gets us to Beta and VHS etc.
Audio tapes machines are different. Machines would load the tape and turn on the electronics when paused, but usually hold the pinch roller away from the capstan so that the tape did not move. Releasing pause and the pinch roller pinched the tape and the tape rolled. Reel to reel recorders had pause even in the early 60’s. I found Sony cassette recorders with pause back in 1971. However none use the pause symbol. Even in 1976, new Sony cassette machine designs did not use the symbol. But in 1977 new Sony machine designs start to use the double bar symbol.
One wonders if might be a coincidence with the introduction of the 1" VTR format.
The gap 1976 to 1977 seems crucial. But it was Hitachi that seemed to beat Ampex and Sony with their Type C VTR. Someone in this gap thought up the symbol. But there is not much to support it being Ampex.
tysm for the technical insight! I’ve been looking into the reel to reel/cassette side of things mostly. I’ve been trying to find out who filed the first patent for a pause function on a magnetic tape recorder, thinking that might correlate to the first use of the symbol. But it gets a little tricky cause lots of companies filed their own patents, and the function was called a “quick stop” at first, not pause. The earliest patent I’ve found was by Telefunken in 1956, but I have yet to see a telefunken device from the 50’s or 60’s with a double bar pause.
The earliest instance i’ve seen of the double-bar pause is in on a JVC 660U which hit the market in 1970. But I feel like there must be an earlier instance than this somewhere because… 10 whole years after the pause function was invented, we finally get the pause symbol? Really?
It became a lot more popular by the late 70’s/early 80s, but did not become official industry standard until 2004 (if i remember correctly, it mightve been 99). The other media control symbols were standardized long before that, I’ve seen them in an ISO publication from 1973, but pause wasn’t there. what all that means is that you could basically use whatever you wanted for your pause symbol. Sony and Philips used a down arrow/triangle pretty often. other companies just wrote “pause” or made something up. This is my favorite example of non-standard symbols on a recorder. Just look at those buttons lol.
But yeah fun fact! that weird circular pause symbol you mentioned actually became ISO standard before the double bar pause. it’s not very popular today though.
I know I’m more than thirteen years late here, but since we’re reviving this thread and this bit hasn’t been addressed, I might as well chip in. I’ve always assumed that the red dot for recording was inspired by the red warning light that has, for a long time, been used in live broadcasting (both radio and TV) to indicate that a camera or microphone was on air. I have no cites to back this up, I’ve simply assumed it because I, personally, found it obvious.