Bay II Filter on Bay I Camera

I just received in the mail what I believed to be a Rolleisoft 1 Bay I filter for my Yashicaflex “New” B (Yashica D), which turns out to be a mislabeled Bay II Duto filter.

However, I realized that when flipped over, the filter fits reasonably well around the outside of the Bay I mount.

Is this a bad idea? Will the filter still function properly in reverse?

Hi frannyg, I would return it for a refund since it was not the correct size and get an actual BAY-1 filter. The BAY-2 filter will not work if you're
using the camera with the BAY-1 lens hood. It can also be annoying having to hold the filter over the lens to take a picture. There are plenty
of BAY-1 DUTO filters available on Ebay at this time. Just my 2 cents...

Return it and get a fitting Rolleisoft. Perhaps the seller confused the Rolleisoft 1 with Bayonet I. They come in two strenghts, 0 and 1. If it matters, all the Rolleisoft filters are coated, but most Rollei - Duto filters are pre-war and uncoated.

With the filter mounted the correct way you can use it with another filter, use it on top of a close-up lens, and with the lens hood. Don't stop the lens down too much when using these soft filters or circles might show up on the photos. I think Franke & Heidecke recommended not going smaller than 5,6 with the Rolleisoft 1.