16.0 and 16.1 should be the bore diameter in MM's, which equates to around .629". That is in between nominal 20 gauge, .615, and 16 gauge which is nominally .66. It sounds from this that you might have a 16 with M/M chokes.
However, a 20 can range from nominal .615 tp to .635.
The 76 is the chamber length in MM's, which is 3"
Clear pics would help