Funny this topic was asked, brought back an old memory... My first job at 16 in the mid 70s (other than paper route and cutting grass) was at a Wendy's. At the time, the advertising message was that all burgers were made to order, you picked the size of the burger, single/double/triple with each piece of meat a 1/4 pounder. Then you ordered your condiments. There was a specific order they were to be put on, and things were lined up in the prep area to facilitate. While I don't remember the specifics, a google search did find the following:
On top of the meat, spread the mustard, then place the lettuce, onion, tomato, and pickles, in that order.
I do remember "mustard on the meat", and that list doesn't include mayo or ketchup. Not sure but one of those two might have gone on the top bun.
Couple of other tidbits... Burgers were always on the grill, to have some available, and if you got one over cooked during a slow time of day without an order, you tossed it in a bin under the grill for chili meat. Chili was made fresh each day with the chili meat from the previous day.
And when you were working, you could put a note on your timecard if you ate a meal during your shift. Then they withheld a small amount, I believe it was $0.40, per meal, no matter what you ate. Being a typical teenage boy with a big appetite, I would do a double or triple and a boatload of fries. My mom told me it was cheaper than feeding me at home and would reimburse me the $0.40.