CRL Tubing Adapters allow 1-1/4" (31.7 mm) Standoffs and Slot Mount Standoffs to be attached to 1-1/2" (38.1 mm) or 2" (50.8 mm) Post Tubing. Use a ZRNUT Rivet-Nut for securing the Standoff to the Tubing.
Image shows adapter with and without a standoff. Standoff not included.

CHECK STOCK with the following links:

1 1/2" Brass - STA112BR
1 1/2" Chrome - STA112CH
1 1/2" Statin Chrome - STA112SC
1 1/2" Brushed Stainless Steel - STA112BS
2" Brass - STA2BR
2" Chrome - STA2CH
2" Satin Chrome - STA2SC
2" Brushed Stainless Steel - STA2BS