A Holland and Holland.......sweet!
Nice group you shot.

Some things you left out of your post:
- PICTURES!
OK, seriously
- May we assume your rifle is a .500 Express (either 3” or 3 1/4”)?
- have you done a chamber cast to confirm the 3”, and/or slugged the bores?
- you shot it with smokeless? ie: Nitro for Black

Those proof marks are standard London proof from the black powder era; View, Provisional, and Definitive.
The “40” is the “bore” proof measurement in gauge (.488”). A very tight .50 caliber by modern standards.
The “S” I have no idea without pictures.

If you have not slugged or otherwise actually measured the groove-groove diameter of your barrels it would be a good idea to do so. Your barrels might be very undersized for shooting standard diameter .510 Woodleigh’s. This would have nothing to do with whether the rounds chamber as loaded because the chambers in those days were often quite generous in dimension due in part to the old coiled brass cases that were available even when there was newer drawn brass case ammunition available.

Certainly possible that a replacement set of barrels was made in 3”.
The 5 Dram marking along with the 3-1/4? Well, Holland’s may have done a special 5 Dram load regulation. Bespoke loads were common to different gunmakers back then.
It would be worth contacting H&H to find out what they have in their records.

Oh, BTW: the group here loves pictures, I does not really exist without pictures!

- Mike