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M42-M42(17-31) M42 to M42 Mount Lens Focusing Helicoid Adapter Ring 17mm-31mm Macro Extension Tube - Walmart.com
Camera Focusing Helicoid Ring Adapter Helicoid Ring Adapter for Mount Lens | eBay
M42-m39 10-15 M42 To M42 Mount Focusing Helicoid Ring Adapter 10mm-15mm Macro Extension Tube - Lens Adapter - AliExpress
l39 N/Z macro Focusing Helicoid Adapter ring tube for voigtlander L39 39mm lens to nikon Z z5 Z6 Z7 z50 N/Z full frame Camera|Lens Adapter| - AliExpress
Repair: Helicoids | Richard Haw's Classic Nikon Repair and Review