That does not look like any rootstock I've heard of - a mystery. Looks like it might be prickly? Most rootstocks are not, for obvious reasons. Do you remember where you bought the rose from?

A local nursery but a long time ago. I did search and couldn't find any yellow rose root stock info. A puzzle for sure. :(

The major rootstock that is used in the rose industry is Dr. Huey. It is a small single bloom, red color with a yellow center. One other one that isn't used much any more is Multiflora. It is very small white blooms that are in a cluster. I'm not sure what color Fortunia is.

Fortuniana is white with lots of fluffy petals, not yellow. I wonder if some other rose (yellow), long gone finally came back from its own roots there.

As an aside, you may have been sold 'Redoute' instead of 'Winchester Cathedral'. Both are color sports of Mary Rose. 'Redoute' was the pale pink version.

I have had roses revert to Dr Huey. This rose is in a garden I created and appears to be on the same plant as 8 originally planted.

Thank you all for your ideas.

I think you are right! I remember googling way back when it first bloomed. It's in an all white flower garden so it was upsetting but I didn't get rid of it because the Austin roses are beautiful. :)