Simple explanation. By advent of the nitro era (1890s), the London trade was no longer attempting to build boxlock DRs, because they couldn't make a profit on them building them in house. Almost without exception, these were "bought in" from the trade - Birmingham - where there were a number of express rifle specialists to "the trade". For example, Webley is famous for the high quality double rifles they made back in the day. Well over 90% of Webley's output bears somebody else's name. It's COMMON to see a specific Webley model, like the PHV-1 for example, with Holland & Holland's name on it, and then see the identical rifle bearing an obscure retailers name that nobody's ever heard of, like London Sporting Park, both with Webley serial numbers on the left side of the loop.

Same with Osborne. Osborne was a major "trade maker", the Lancaster in the link above for example. Lyon & Lyon, Calcutta wasn't a gunmaker, they were just one of many retailers scattered around the empire. The guns bearing their name were all made by someone else. Double rifles like this one, "made for the trade", were still hand made, and what was delivered was what was ordered. Caliber, type of fore-end latch, bolted safety, sights, stock dimensions, etc...were specified by order. Few were delivered exactly the same.

The actual maker is sometimes readily identifiable, but sometimes not. That's where the markings come in. The 3 digit number on the trigger guard tang is the retailer's - Lyon & Lyon. As a rule of thumb, the Birmingham trade makers DID mark guns built for the trade with their own serial numbers - there are exceptions, such as Wilkes, another prolific trade maker. The normal practice was to place this number on, or in the immediate vicinity of, the fore-end loop (the stud soldered to the bottom of the barrels that the fore-end attaches to). That appears to be the number you referred to as being in the 40,000 range. That's the number I need.

Given the "40,000 range", there's one more mark that I hope is there. There may be a mark that appears as a pair of crossed swords with a letter in the 12 o'clock angle and a number in the 6 o'clock angle on ONE of the barrel flats. If it's there, what is the letter at 12 o'clock? Take a magnifying glass with you.