Thanks for the load data.
Can you think of a current production 300gr bullet (other than the Barnes X) that's up to the velocity?
I haven't done any kind of research yet as to how much velocity'd be left at three hundred yards with that high MV.
Looks like it's worth a peek...

So, on gunmarks.
The right side of the rear bridge has a four digit number stamped between the sight mount screws -- looks like 0585
Do you think it's a date code?

There is a barrel mounted recoil lug too, I didn't measure the distance from the reciever, but let's say a couple and a half inches or so. The lug itself is a tad crude, the provision for it on the barrel looks good though and I'll likely make a new lug that's a much more precise fit to the recess in the underside of the barrel. The stock definitely will need bedding too, but from the factory there was bedding compound in the stock inletting for the lug. It's all going to get a fresh new outlook on things before I run the rifle.

Other marks that might interest you are for one, MarkX on the left side of the forward bridge, that's between the 'Cal. 458 Win. Mag.' in script on the barrel chamber and 'Alexandria Virginia Interarms Manchester England' flanked by the circle/star stamp to the left and the CZwithin circle logo on the right all together on the left side of the reciever under the bolt/loading/ejection cut.
On the underside of the reciever flat near the forward action bolt hole is a capital letter 'W' and what look to be possibly in smaller characters near it 'LV'
The stock has an underscored 'DR' stamped in the inletted area for the triggerguard between the trigger inletting and the magazine box cut.

I hope that helps you out a bit, at least gives you something to ponder.

--Tinker