It's hard to imagine a 28 on this frame, but I guess it's possible. Here are the measured dims. Sorry for the fractions but I couldn't put my hands on my dial caliper.

Across the flats 1 5/16"
Across breech balls 1 3/4"
Between f. pins 7/8" CTC


Interesting note, the fore end lug is one piece 4 3/16" long. Should be very strong.

SRH


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