Self Centering Visual Back M48-thread to 1.25" with M48 Female Thread
It is practically a 1.25" eyepiece holder (also called visual back) that can be attached to (screwed onto) a male M48 thread.

Specifications

• M48 female (M48x0.75) thread on the telescope side
• Self-Centering 1.25" eyepiece holder barrel on the eyepiece side with teflon compression ring
• Overall length: 33-35mm
• Black anodized base, blue anodised top
• Internal diameter: 31.7mm