Rb67 light leak

I have tried 2 backs with similar results,
If I'm right in assuming that the image is upside down and the emulsion faces the lens then the light leak shows up about an inch wide on the winder side near the edge of the frame.
Strangely the first frame on each roll in different backs was not effected neither was one frame shot in portrait with one back.
Any thoughts appreciated

ps. The fogging is vertical the full height of the frame

With the RB67, you can also get light leaks from the rotating back adapter, or from the darkslide going in and out of the slot. Do you have any example images to help diagnose?

Haven't any images to post at the moment,but I did shoot using and not using darkslide between shots and it didn't make any discernible difference

Darkslide or rotating back light seals. It sounds to me like the darkslide seal. The darkslide needs to be removed to make a photo, so it doesn't matter the sequence of darkslide presence, or not, prior to the photo.

If the camera hasn't been re-sealed, it's time. Any Pro body, and most Pro-S bodies, need this now due to age. There is a large body of experience with this that can be found online. The materials are inexpensive and the work isn't terribly difficult. The felt+cloth darkslide seal on my Pro S needed to be soaked (fully submerged) in alcohol for a day or two. It seemed to have a different adhesive than the sponge/foam light seals on the back and adapter.

+1 for replace the light seals. Only one kit from Japan shows up on ebay, there may be other sources.
In the meantime in total darkness with the roll film holder removed and mirror up shine a bright light in through the back opening and observe the exterior of the body especially at the lens and viewfinder for light leaks.
Next with a roll film holder installed and the mirror up shine the light in through the lens opening observing the body and roll film holders for light leaks. Anything other than total black is a light leak.
The body will absorb the light so you will hardly know the light is there once past the camera opening.
A AA Mini Mag focused to 2 inches or a small LED light is perfect for checking. Point the light at the edges of the mirror, lens, and roll film holder.

I've ordered seals from Jon Goodman for backs, revolving back and mirror... should be the cure.
Thanks for advice everyone