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I am assuming the 15.8 translates to .622" which is slightly oversize for a 20 gauge bore. (.617")
If that is correct, then the 14.8 would indicate about 39 thousands constriction which would be equal to a very tight extra full choke.
... the 15.7 would only be about 3 thou constriction which would be equal to a skeet 1 choke slightly less than improved cylinder.
Weird chokes if that data is correct. I would suggest having the bores measured and the constriction measured.
In 20 gauges - 25 thou constriction is a full choke, 14 thou constriction is a modified choke and 6 thou constriction is an improved cylinder choke.