Very interesting.

I couldn't think of being disappointed with this gun at all if bought within reason.
Actually, I linked to this gun as an example of a sporterised MBL in another thread - http://forums.nitroexpress.com/showflat.php?Cat=0&Number=146215&an=0&page=1#Post146215

I imagine that the barrel could be original and subsequently rebored to 500ex.
If it was originally chambered in 461 No.1 Gibbs, it would have been rifled in the metford system and so marked on the barrel with a patent use number. Obviously, the original rifling is long gone but the "48" bore marking could indicate that this was originally chambered in something other than a proprietary Gibbs cartridge. The sporting rifles were advertised as being available in .450cal so I suppose a MBL could have been ordered this way as well. This could explain the lack of a Metford number - Winfer claims that only the Gibbs chamberings had Metford rifling.
Now, would Jeffery roll their name and address onto a barrel which they simply rebored? I dunno...

The forend is a bit Webley looking. The classic Jeffery style is upswept as finished by Turner although if you had a gun "sporterised" in period, I bet you could specify whatever forend you liked. The checkering looks to be more coarse on the forend than the grip area and I don't know if running the checkering all the way back to the action was done by any maker in particular.

As I said, very interesting rifle.
IMO, since George Gibbs Farquharsons in absolute original condition are so damn rare, it is still a treat indeed to own a genuine Gibbs Farq of any configuration, no matter how mysterious.