Bottom bracket shell mark ID
 

The C&V forum suggested I reach out to framebuilders for my question about bottom bracket shell sources. I have a late 80s - early 90s Giordana frame. Most Giordana frames are attributed to Billato. The frame size and tubing is stamped into the bottom bracket shell - “58 SL”. The letters “SR” are cast onto the shell surface. I’m curious about the source of this shell. Sakae Ringyo seems unlikely.
Any ideas?
https://cimg6.ibsrv.net/gimg/bikefor...4c761d31a.jpeg
https://cimg0.ibsrv.net/gimg/bikefor...cb6c34660.jpeg
https://cimg1.ibsrv.net/gimg/bikefor...be5eddf64.jpeg
https://cimg2.ibsrv.net/gimg/bikefor...8d643dcfe.jpeg