Maybe even convention is too strong a term, but back in the '80's, when I personally started seeing these gadgets appear in all sorts of applications, the 14-pin connectors, of both types, of course, were the ones I saw in almost every application.

I've seen it go both ways with the SIL connector, i.e. right-angle, typically, on either side of the board. Without the right angle pins on the board, the connector can make the module somewhat bulky, and I've seen enough of them done "wrong", i.e. in the way I think we'll both agree they shouldn't be done, with the cable connector on the glass side of the module, that I had to point out that that particular type of connector assembly wouldn't work well without the right angle pins. With the SIL arrangement, it merely requires that one place the connector on the side opposite the display, and no respin of the board is necessary.

You're right, particularly in that there's really no applicable standard, though for years the overwhelming majority of connectors has been of the 2x7 or 1x14 type. What's always puzzled me is that I've not seen any effort at labeling the connector pins in the silkscreen so one would know what the purpose of the extra pins might be. As I said, I'vee seen 'em applied in a number of ways, not the not the least of which was power for the backlight.

RE