They are "stacked" copies of the original you are working on.
This happens when you use an external editor such as silver fx/dfine where you choose to edit in the external editor with either a copy or a copy with Lightroom adjustments.
External editors usually require a "pixel" file to work on ... JPG, PSD or TIF and these are what are returned and stacked with the original RAW file.
A couple of references for you:
Many thanks, BKKDon, for your prompt reply. I’ve looked at the links you sent, but I’m still confused. The link re stacking refers to groups of similar photos. I’m getting a stack of the same photo. Is the raw original the one marked e.g. “1 of 3”, and is the latest one that I’ve worked on “3 of 3”? If not, how do I know which is which?
The mention of the fact that they are "similar" photos is simply if you stack them yourself. That link was so you understood that stacks are stacks of images in the same folder.
Each time you call an external editor the result is re-imported to Lightroom and stacked with the original ... usually a RAW photo. For example, if you do edits in LR and then send the image to Silver Efex Pro a TIFF file will be created for export with the LR adjustments and then when you save the result in Silver Efex Pro the subsequent B&W file is re-imported and stacked with the original RAW file.
It is a way to organize the files and flag that they are linked ... typically "1" refers to the first image in the stack and therefore it is the last created. So if I had a RAW file then exported to Silver Efex Pro then exported a copy to dfine they would be stacked in the following order ... dfine - 1, Silver efex pro - 2 , original - 3 ... the original is always the last in a stack and the latest you have worked on is always 1.
Very many thanks again for a prompt and very helpful reply. I now understand the process.