I don't think any of my enlarging lenses came with rear caps because of the packaging they were sold in. They were screwed into a plastic base with a clear top, hence the need for only a front cap. I have a couple of these that came in darkroom lots I've picked up over the years.
Yeah, I've got a few of those as well. They aren't very useful though after the lens is mounted.
Still seems front and rear caps could be pretty useful to keep the dust off and such.