I came across “Idaho Ordnance Factory” brand ammo today, and they have at least one custom headstamp - in 9mm. Apparently the name “Idaho Ordnance” was already taken, and so this outfit in Dalton Gardens Idaho adds the word Factory to have a separate brand. I came across them because they are one of a growing number of smaller manufacturers using the Maker Bullet P-REX and T-REX solid copper hollow points.
The 9mm headstamp is IOF 9mm LUGER which I only saw in a video dated Oct 29th of this year. In the screen-grab image below the headstamp on the left looks like “IGF”, but you can see that it is IOF on the one on the right, which makes sense for their brand name. The best price online (and only retailer of it other than IOF direct) is Axis mfg, who list the 9mm P-REX 20rd box for $20.00. They are a new company anyway, begun in 2017 as far as I can tell. Hopefully they do a .300 Blackout headstamp!