Today’s Question: Can the reference image [in the Develop module in Lightroom Classic] be a prior version in the history list?
Tim’s Quick Answer: Yes, if you use a virtual copy to represent the prior history state for the image.
More Detail: As noted in my answer from November 8th (https://asktimgrey.com/2023/11/08/reference-view-for-optimizing), you can use the Reference View feature in Lightroom Classic to have a reference image displayed next to the image you’re working on in the Develop module. This enables you to apply adjustments based on a different image, such as when you want to match the overall look of one image for another image.
Any image can be used as a reference image in Lightroom Classic, by simply switching to the Reference View and dragging an image from the filmstrip on the bottom panel into the Reference image area.
If you want to use a specific history state for an image as the reference image, I suggest creating a virtual copy for this purpose. Start by selecting the image in the Develop module and selecting the applicable state in the History section on the left panel (or select a snapshot if you’ve created one). Then right-click on the image and choose “Create Virtual Copy” from the popup menu. The virtual copy will inherit the current settings for the image, so it will reflect the history state. You can then set the original image back to the latest history state at the top of the History section.
The virtual copy created based on a specific history state for an image can then be dragged into the Reference image area. This provides the effect of having a specific history state for an image for reference while working on another image in the Develop module.