This is an Ealing mounting blank disk. It is threaded for use with components that are outfitted with the 5 inch 12 TPI standard mounting ring as found on their Research Precision Optical Bench. The disk does have a threaded hole in the center (as supplied from Ealing, it would have only had a small pilot countersunk hole). There are also a couple of small counterbored holes in the disk..
***MPN 23-1787
***OEM Ealing / Beck
***SKU*** DP217



