The 61-4 from BrassCraft is a 1/4” OD Compression Brass Nut. It's used in combination with 1/4" OD compression fitting or outlet and 1/4" OD tubing (copper or plastic). Sleeve (ring) sold separately. Made in the USA.

Technical Specifications:

• For use with: 1/4" OD (1/8" Nominal/CTS) tubing and compression fittings;
• CSA Listed, NSF/ANSI 372;
• Made in the USA.