is there really much difference between a rear brake light and rear fog?
Yes. The minimum axial intensity (at a single test point straight out from the stop ("brake") lamp is 80 candela per SAE J586. The minimum intensity for a rear fog lamp, per SAE J1319, is 125 candela along the horizontal line from 10 degrees Left to 10 degrees Right, and along the vertical line from 5 degrees Up to 5 degrees Down (these lines cross at the axial point mentioned above). The intensity must not be less than 62.5 candela everywhere in the region ("zone") defined by the diamond shape formed by the endpoints of these lines. That's the upper diagram in this attached picture. The lower diagram is the stop light specification, and as you can see it's a lot more complex. The circled numbers are the minimum candela values at specific test points, and the pink shaded zones have their own requirements, too -- it's not enough to just meet the test points; the zones also have to be fulfilled. For both the rear fog lamp and the stop lamp, the maximum intensity everywhere in the light distribution is 300 candela. Note that these two specifications are not mutually exclusive/incompatible; it is possible to make a single lamp that meets both the stop and rear fog functions. But meeting one of the specs doesn't guarantee meeting the other one.
Also doesn't the SAE spec say it should say F2 on the lens
Function markings like "F2" for a rear fog lamp or "S" for a stop lamp are not legally required, and they actually aren't even required by the SAE standards themselves. J586, for example, says stop lamps "may" be identified by the code 'S'.
But if we look at this from Hella that doesn't even seem available here in the states
That's a fine one. Everything from Hella is available here, you just need to know where to look! You can either go it alone (put the Hella part number in a search on Google, Amazon, or Ebay), or check with Daniel Stern or another reputable vendor.
That's another good choice, and probably
faster/easier to buy here. There are numerous brackets available to mount these lamps, such as
these,
these and
these. There are round versions of these lamps/brackets, too.
If you go to install a rear fog lamp, or a pair of them, keep in mind the hookup and switching arrangements need to be appropriate, not just a simple on-off switch. Rear fog lamps can cause serious glare and other safety problems (stop lamp masking) if they're indiscriminately used or forgotten and left on. Factory installations have to be set up so the rear fogs can be switched on only if the front fogs or headlamps are on, and if the headlamps/front fogs and/or the ignition is switched off, the rear fogs also have to go off
and stay off until manually activated by the driver. There's a nice little
kit available to make it easy.
what is the real difference between these and the very similar looking "junk" sold on ebay or amazon as "fog" lights?
The reputable-brand items are engineered, designed, manufactured, tested, and certified (and/or approved) as legitimate safety devices. Off-brand (or no-brand) items...aren't. Again, it's not just a red light, there's much more to it than that.
