400NE:
Your assumptions on the weight will be correct for sure, the gun is invariably a BP Express. Pre-WW2 British-built .500 Nitros under 11 lbs simply do not exist.

9 3/4 lbs is the industry-standard 'correct' weight for a .500 Express. Obviously you already know this, Hollis500, hence your date estimate of 1870 to 1900.

Does this suggest a break in sequential numbering for the Hollis guns? A pity the records are not available.

Certainly a photo or two would help, especially the proof-marks on the barrel-flats as suggested. Any extras like whether ULH, TLH, or hammerless, if a hammer-gun whether the action is rebounding, and whether the hammers are bolted or not, might help narrow-down whether it was an 'early' or 'late' period gun.