Mystery light leak or mottle with antique plate holders with film sheaths

So I processed some TMAX-400 I shot two weeks ago in a Rochester Optical 4x5 Pony Premo No. 4, using period plate holders with Kodak film sheaths. Some shots have a strange mottled fog pattern, as do two that I didn't expose.

I'll start with one shot displaying the leak on an image.

The leak is in the lower right area of the photo. This is the end of the plate holder where the dark slide comes out. Not fully trusting the wood flaps that are the light traps on these holders, I keep the dark cloth over the camera while the dark slide is out. Maybe the dark cloth isn't that opaque, but when I've had those leaks they have been straight lines streaking in from the edge of the film.

I've held up the dark slides to bright light, they really appear to be opaque. Plus the problem is always in the same corner.

This is more of a mottle. Maybe it's chemical? I diluted the HC-110 from syrup into the Nikor 4x5 tank, so perhaps it wasn't well-enough mixed and this area got "strong" developer. I then lowered the reel into the full tank inside my changing bag. 5:15 in HC-110 dilution B at 68F.

Here's the first "blank" sheet. There's a slight "real" light leak on the left. Can't blame the camera, the dark slide was never pulled except in the changing bag.

The scanning exaggerates the degree of the "fog".

Here's the second "blank" sheet. Maybe there's a slight "real" light leak at the bottom.

The end of the sheet with the mottle was towards the outside of the Nikor reel. If I've got my spatial geometry right, it was also on the upper half of the reel.

Maybe I should stick with pouring the developer in through the lid. But with the short developing times required by HC-110 dilution B, I wanted to aim for the best uniformity. I do have a quick-fill Q50 tank that I put the reel in, rather than the old-style tank the reel came with. I didn't have this problem with the film from the Tri-X Professional 4x5 TXP523 film pack I also shot that weekend, where I poured the developer in through the lid. But that was a 12:30 developing time.

Oh, the TMAX-400 expired in 2015, kept cool but not refrigerated. There is some base fog.

As for why I processed film I didn't expose, I had put in one dark slide back the wrong way after exposing, so I no longer knew which sheets were already exposed. Chose to waste two sheets of film rather than do an un-planned double-exposure.

There's a strong possiblity that the film was fogged before it even went into the darkslide.

Try developing a sheet straight from box to tank.

I'd also suspect your unusual 'dilution' method. Why not mix the developer properly in a measuring cylinder before tipping it into the tank? Even if you then dunk the reel into the filled tank.

Also, a 5 minute developing time isn't that short, and the few extra seconds it takes to fill and empty the tank will make almost no difference.

Can't be any problem with the camera, the dark slides were never removed from the plate holder of the two "unexposed" frames.

Not a problem with the film from the box, processed one sheet today along with the last two exposed sheets. I mixed the developer in a polypropylene pitcher. I made sure to mix it very well. I poured the developer through the fill spout of the quick-fill Q50 tank, and had it at 65F so that I could use a 6:15 processing time. Everything looks fine.