Let me add my heartiest congratulations to those earlier written by the other correspondents. Yours is a lovely, early hammergun by Harris Holland (i.e. "H. Holland," gunmaker as of 1835; Henry Holland was the nephew and one-time apprentice of Harris Holland, whose common surname was joined to the later established firm of Holland & Holland), which we are delighted to learn you intend to use afield and / or at the shooting club. The choke arrangement, as has also been suggested by others here, is standard for driven game shooting. Several of my antique, U.K.-produced guns have this same configuration.
From the photographs, it appears the nose of either hammer does not strike the firing pin or detonator and that it is instead struck by a metal stud affixed at the offside of the hammer breast, which if I have correctly described it, is an interesting mechanical arrangement. Though I do not think it unique, as I recall seeing this arrangement employed before and used by other period makers. The presence of its Jones under-lever locking system is a definite plus and one in which you can have confidence.
It seems unusual for a gun from this era to have original 2-3/4 inch chambers, although not impossible, unless they were later lengthened. But if lengthened properly, and if adequate chamber wall thicknesses remained following the work, should serve. Your choice of low-pressure loads is sensible. I shoot all of the my antique guns and only employ similar low-pressure ammunition, at standard of the era velocities, and with no more than standard weight charges of lead shot, which has proven more than appropriate for my shooting.
Enjoy your wonderful Harris Holland gun to the fullest; it is an excellent example of the gunmaker's art!
Regards,
Edwardian
