1018 low carbon steel material tapped 8-32 with countersunk coil screw holes. Designed to be used with 7/8” height cores or smaller. 

As you can see in the first pic measurements these are taller than a normal base at 1”.  Please be advised that standard length needles may not be long enough to accommodate these bases, you may need to buy longer ones. You may be able to also switch to a really short tube grip to offset the height difference.  

These have been sitting around in boxes for about 10 years. Some have a bit of surface rust that will easily clean off. 

These are sold as is. No returns. Ask questions before buying.