OK this is why i ususally pay a professional plumber to do this sort of thing. I removed the screw-in drain flange and discovered that indeed there was a part where the putty had slid over and bunched up, leaving a gap in the seal. I applied LOTS more putty, and reinstalled the flange, and leak tested it with a 1/2 full tub. NO MORE LEAKS!!!
Word to the wise, from the dumb and inexperienced: Always use more putty than you possibly think you will need. let the excess squish out (technical term I think). Thanks for listening. I'll be back when I shart the kitchen this summer, unless something happens between now and then Which, with a 78 year old house is always possible!