It seems like Solum Tool, beloved supplier of various HK handgun magazine extensions, has been out of stock on like half their items for quite some time.
I emailed them to inquire, but got no response. Anyone have any thoughts on this?

I've noticed that as well.  I checked their Instagram page and the last post was in October.   I hope they are still making products because I really enjoy them.

It took me twice using their website messenger to get a response, and had to mention I'd reached out months prior and not heard back.  He apologized and said he would be manufacturing again but was slow. He said the best way to find out about restocks was to sign up for the newsletter.  That was about a month ago and haven't received a newsletter since.  I think his 45 mag bases are most popular.  15 rds in a 45 usp is a big deal.  I'm hoping that's what he's making next!