I also remember their terrific sauce. I feel your pain.
Some of you guys on here probably remember Brockle's (sp) salad dressing, also.
I found the recipe for that too. But, part of the problem with reproducing it at home, has to do with the fact that it was produced for them, at a commercial kitchen/bottler, so it was blended without the egg breaking down like it will when you use prepared Mayo at home. With prepared Mayo, you can't blend it to smooth out the blue cheese...
I mention this, because I have a suspicion that the same thing was going on with Underwood's and Stern's BBQ sauce.
See the Stern's thread where the owner declined to give out the recipe because he was afraid it couldn't be reproduced at home--I think they may have been using Lecithin to keep the sauce from separating from the drippings fat.
I suspect that sometime back when, in the 40's or so, the original Underwood discovered that the addition of little mayo, smoothed the tangyness of the tomato and also had the side effect of keeping the fats and non-fats bound together. I doubt a BBQ man back then knew much about Lecithin, but could be...
Anyway, that is my suspicion. They had a commercial bottler making the sauce base to be blended with drippings at the store.