Abbatron (H.H.Smith) #120 makes a Shaft Coupler, for 1/4” shafts.
https://www.abbatron.com/uploads/pdf/SD-120.pdfKeystone Electronics (New Hyde Park, NY) sells a Variety of Assembly Hardware used in Radios & Electronics since World War 2
Shaft Coupler, Nickel plated Brass, 1/4” : #1453
https://www.keyelco.com/product.cfm/product_id/1592Allied Electronics, Digi-Key carry these parts==
WARNING: METRIC (6mm) shaft couplers are prevalent outside North America.
Aluminum, Brass, and Steel are more common than Composites, Plastics, or Nylon.=
Small Parts, Inc., joined the Amazon family in 2005 and continues to sell more than 10,000 items branded as SmallParts that maintain the same high quality standards. You'll find Small Parts products in tubing, fasteners, raw materials, and more.Since Amazon bought Small Parts, Inc. (Florida) in 2005.
you should be able to find a suitable replacement by Searching Amazon.
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COMPONENT SUPPLY COMPANY (Sparta, TN and Fort Meade, FL)
has filled the Market Niche, formerly served by Small Parts 20 years ago.
https://componentsupplycompany.com/