20-76 indicates a 20ga gun with 76mm (3 inch) chambers.
15.3/15.9 and 14.8/15.9 are how the Basques indicate chole. The smaller number is the actual muzzle diameter in millimeters and the larger number the bore diameter. If you subtract the smaller number from the the larger and divide by 25.4 you'll get the constriction in inches.
15.9-15.3 = .6 .6/25.4 = .024 inches or about 3/4 or IM choke in 20ga.
15.8-14.8=1.1 1.1/25.4 = .043 inches or a really tight full choke.
You should also see a letter and number combination, something like M1 or P1, or something like that. This is the date of manufacture code for all Spanish guns built before 1994 (I think). You can find the exact date that matches your code here: http://personales.jet.es/rafa/b_punzone ... fecha.html