GIMP Version: 2.10-28-1
Operating System: Linux
GIMP Experience: Intermediate Level

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The attached image is a small detail scanned in from a print made in 1917 and put in an album something over a hundred years ago. I'm not even sure what to call the shmutz over the image. I doubt that there's a de-shmutzing filter. Anyone have an idea of one that might work? Thanks. - Jeremy

This is the first time I've tried to include an image. Not sure it's going to work. Thanks for your patience.

It might be better changing the mode from greyscale to RGB: Image -> Mode -> RGB

However even as greyscale, try Colors -> Levels and click on 'Auto Input Levels`

Looks like lots a dust to repair, some good 'fixers' on this forum sure to give advice.

Can you give at least a proper/larger portion of that scan, we cannot know on what we are working, is the shadow on the edge (of a window?) a hinge of some blinds/shades?
I do need a proper portion of that photo (even better with people on it) to validate my tries as there is not only shmutz but also big pixels from a huge jpg compression

Thank you all for the help!
SallyAnne - You get significantly better wavelet filter results than I do. I'm using GIMP on Linux, not Windows. Doesn't seem that should make a difference. Is there a combination of sliders and flags that you favor?
- Jeremy

Sorry Jeremy. I have never been able to work with it but others on here swear by it. What I did was all manual. Actually if your photo is that old it probably has mildew all over it and I havn't found a filter yet that will remove it.

Not too many suggestions so far so my 2 euros worth PixLab has a point about how representative the clip is, what works here might give different results for a whole image. However (always a however)

A look at the clip properties and the jpeg quality is very low, 20 This might just be the clip and the original might be OK.
Jpeg quality settings are not linear. There is a big file size saving for very little loss in quality 95 down to about 80 but 50 and below the other way round, big loss in quality for very little reduction in file size.

The file resolution is 2400x2400 pixels-per-inch (ppi) This from a scan setting ? Photo quality generally taken as 300 ppi, no great benefit in scanning over 600 ppi. It is up to you.

What linux are you using, and what 'flavour' of Gimp - from repository / flatpack / snap / PPA / appimage (edit 2.10.28 not appimage) ? It will make a difference when it comes to available 3rd party plugins.

Using the gimp_gmic_qt plugin see: http://www.gmic.eu and a 3 minute video on how I might proceed https://youtu.be/jKcWyFCz4Mg

Your window frame looks like stucco! Even if it is not the real look, it looks good!
But I think tenforty should provide a bigger section