ZWO M48 (male) - M48 (female) 16.5mm Adapter - ZWO-M48-EXT16.5

The ZWO M48 male to M48 female 16.5mm adapter can be used to obtain precise spacing when using a focal reducer or corrector, or other accessory in order to achieve the correct back focus.

The M48-EXT16.5 is made of black anodized aluminum.  You could also use it as a short nosepiece, or as a M48 general extension tube to add 16.5 mm of optical length to your set up.