I found this while going through some old test results. It is a latent image stability test. After the film is exposed and before it is developed, silver atoms, particularly in low exposure areas, have a tendency to lose electrons and revert back to an undevelopable state. This has a tendency to lower film speeds but also has an affect on overall contrast. Most of this loss happens within moments of exposure and then plateaus off over time. Storage temperature of the exposure film can play a part of the rate of the regression of the latent image.

The ISO speed standards have a "hold time" incorporated into the standards. It was originally two hours, but with ISO 6 it changed to 4 hours to 7 days for professional B&W film and 5 to 10 days for general purpose film to better reflect real world usage.

The test I conducted exposed sheets of 4x5 film over the course of a month using a calibrated EG&G Mark VII Sensitometer. All the film was processed at the same time.

Latent Image Keeping Test
Hold Time EFS
Test 1
30 sec 94
1 hour 78
2 hours 78
3 hours 78
4 hours 78
1 day 74
2 days 71
5 days 71
7 days 71
14 days 68
21 days 68
31 days 65

Test 2
30 sec 89
1 hour 85
2 hours 78
4 hours 75