Maybe I'm blind but I can't find patch notes for 0.1c or 0.1d. Are patch notes not a thing yet? They're extremely helpful.

The only thing that exsits is the "Changelog" button on the download page, but the first two versions have no notes, and the second two have "Updated User Documentations" which is hardly a version hotfix, (no reason to go from '0.1b' to '0.1d' if you just changed the manual twice.).

I don't believe anything in the mod was changed, just what's included in the download. The letter changed because it is a different release and is easier to keep track of.

 

At the bottom of that page you can see What's New in Version 0.1d Beta  and next to it in the grey area you can change the updatenr, and below it will show what was changed in that update. Mostly user documentation in this case.

I believe in 0.1c (might have been 0.1d) it was made so that LSPDFR automatically loaded, rather than having to type ' loadplugin "LSPD First Reponse.dll" '. The others, as Will said, are because there was still a difference and it's easier to track that way.