B-51-03-02-03 - 3/16" x 1/8" 
B-51-04-02-04 - 1/4" x 1/8" 
B-51-04-04-04 - 1/4" x 1/4"  
B-51-05-02-05 - 5/16" x 1/8"
B-51-05-04-05 - 5/16" x 1/4"
B-51-06-04-06 - 3/8" x 1/4"  
B-51-06-06-06 - 3/8" x 3/8"  
B-51-08-06-08 - 1/2" x 3/8"  
B-51-08-08-08 - 1/2" x 1/2"  
B-51-10-08-10 - 5/8" x 1/2"
B-51-12-12-12 - 3/4" x 3/4"

This brass hydraulic SAE 45° fitting is equivalent to the following:
-SAE 010424
-Eaton Weatherhead 51