Breaking news, July 3,2015:
Duralast RB401

Finally it looks like we have a winner. Duralast RB401, Made in Spain, purchased at Autozone.

Installed July 3 2015, still working December, 2016,
17 months and 50,000 miles in service.
This one has Lucar style spade terminals.

Autozone does have a part number for a Duralast switch with screw post terminals (as original for MGA), SW8048 (but I don't know yet if it is also Made in Spain).

There are two more switches (other brands) I would like to try, but I'm not removing this one as long as it is still working.

Addendum: It finally died, March 15, 2019 after 3-yr 8-1/2 mo, 130,204 miles in service.

Addendum 1/28/2018:
This may be bad news. The Duralast RB402 switch in my car for 2-1/2 years, 94,000 miles, is still doing well. I just bought another one and cut the new one open to seee what was inside that makes it so good. Unfortunately, the result was discovery of the same slow-make slow-break type contacts as found in all of the known bad switches.
This one is FAE 21020, which is a spade terminal version of the FAE 20010 screw post switch which was previously determined to be bad. In fact it was just one week earier that I removed (and opened) the 20010 switch that failed after just 250 miles. I now have to suspect that Autozone has changed its source and is now vending bad switches packaged under the same brand name and part number as the previously good switches.
Now I will have to take the one out of my car and check the numbers underneath, since I did not record them when it was installed. Navigator is not going to let me cut open the good switch in my car (which is why we bought the new one). If it turns out to be a different manufacturer and part number, that would be good news, as we can look for another source for the good switch. If it turns out to be the same manufacturer and part number, that would be (almost unbelievable) bad news, figuring the maker may have changed from good design to bad design. I don't know how else to explain it (yet).

Addendum 4/4/2018:
Keith Meyer wrote:
"I bought a Duralast RB 401 from Autozone for my early MGB GT. It was the last one in that store. This version had all the references you mentioned stamped on the underside of the switch. FAE; 21020; said Made in Spain, ect. It also had no sealant on the threads. I decided to buy another for backup. The store imported another RB401 from a store about 20 miles away. This RB401 switch is very different. It has no markings on it, it has a white top with no “wall” between the male spades, the spades just protrude from the plastic, its point of manufacture is not identified, it has a sealant on the threads. It does have a disclaimer saying it is not compatible with silicone brake fluid the first RB401 did not. So, it looks like there is now a third version of the RB401. Since I bought it for backup I cannot report on its quality". -- Keith

So now it looks like they have gone from good to bad to worse than bad. Looks like Keith is going to be the test rabbit this time. Stay tuned.

And this one August 24, 2022. Same manufacturer, FAE, part number 21020 with black insulator and spade terminals, same failure with burned contacts.