Have been emailing back and forth with the affable Mr Patrick Hawes of Bonham's.
Inquiries on my end started as the Lot was "Withdrawn" right before the auction. Turns out that on further inspection it was deemed that the bolt was a replacement and that the bolt proper did not carry a Proof marking.

Proof could not be carried out on a timely manner for the auction. It is recorded by Holland's as a 400/375, of which we of the knowing/assuming thing that is not possible in a pattern 92 action. The en-bloc clips will not secure a 400/375 where as they work just as designed with a 375 2 1/2" NE.

Yesterday it was chamber checked and is indeed a 375 2 1/2" NE and pasted proof. It will be back in the catalog for the July sale.

As I will be a few months into gainful employement by the Gavel time, I am thinking I may make a run at it. A Man could own worse rifles to be assured.