Considering it looks like it was written with a fountain pen, it's probably from the same generation as the book. If you can't make out the writing, this is the recipe:
2 eggs, well beaten
1 teaspoon dry mustard
1/4 cup sugar
1/2 tsp. salt
Dash of red pepper
1/2 cup vinegar
Piece of butter the size of small egg
Put on stove in double boiler and stir until it thickens.
And that's it. No other instructions, and no hints as to what sort of salad this is supposed to dress.
So, okay, I figured it was worth a try.
I wasn't sure exactly what sort of red pepper was required, so I took the mild way out and used paprika. I figured I could spice it up later. And I wasn't sure how small that small egg of butter was, so I just used up what was left in the butter dish. That's about as vague as the small egg. Maybe 4 tablespoons.
I put the resulting mixture through a strainer to catch any coagulated bits, the same as I would any other stovetop custard-like item.
Right off the stove, it was very vinegary. I used white vinegar because I figured that would be most common type of vinegar back then. After it cooled, it was still pretty vinegary. I probably would have been fine as a base for some sort of dressing, but on its own it was too tart. And I like tart. It needed a bit of adjusting.
So I added more butter and sugar, and beat up another egg. I heated the mixture a bit, tempered my new egg, put it all back in the pot and let it thicken again. For the fun of it, I added a bit more paprika, too. And let it cool.
Now, it's a lot thicker; just about the same thickness as mayo. The flavor reminds me of a honey-mustard dressing. This stuff would be great on a ham sandwich. As far as putting it on a green salad, it will need to be loosened up a bit for it to to work well, but that's easy enough.
Next time, I'll use a milder vinegar or maybe cut the amount in half. But it's an interesting recipe.