OK, I've looked at DanTwoLakes attachment on "sewing windlace" thread and he has got a picture of a 3/4" welt foot. I can see that the inside presser foot has an angle on it which would allow the needle to get as close as possible to the curved welt. This could be made from a regular foot if I had to ?.....yes or no ??.
The outside foot has a curve on the inside of it and I guess that the 3/4 refers to the curvature. This one would be harder to make.
Thanks for posting the pic.......now I know what I'm looking for.