I agree that it's most likely 'Veilchenblau', but didn't weigh-in because there wasn't another possibility mentioned. Jeri's reasoning is sound, and finding the various rootstocks "growing wild" does point to the likelihood that there once was a budded rose planted there, but that doesn't eliminate 'Veilchenblau' since it, too, was once used as rootstock, as were several other ramblers. Even my 'Baltimore Belle' has references pointing to it being used as such. I also recall 'Felicite et Perpetue' being praised as rootstock in a 19th Century reference. Back when there were far more small producers of roses, what was used as rootstock could be almost anything that rooted easily and grew vigorously. So, if you happen upon a rambler growing all but forgotten for a very long time in a space that "doesn't make sense" for a rambler, it could very well have been rootstock.
:-)
~Christopher