The Interarms Mark X and Whitworth are the same rifle. They are both Zastava barreled actions, like the Remington 798 was, as well as a number of other brands (e.g. Parker Hale). It is also available from Zastava directly as the M70.
This is a solid rifle.
The only thing to watch for with used Whitworth rifles is that some of them were stocked without enough relief behind the action rear tang and can have hair cracks in the stock.
Nonetheless, I would recommend that you stick to CZ 550 for the following OBJECTIVE advantages:
- true magnum .416 Rigby length action (most other Mauser clones are 30-06 length with a or .375 magazine well);
- double square bridges with integral scope bases (they will never get loose);
- 5 rounds drop belly magazine capacity in .375 (not needed until ... needed);
- drop-forged steel action and 100% bottom metal steel (no risk of pot-metal stress cracks);
- integral rear sight barrel boss (rear sight will never get loose);
- barrel band front sight (front sight will never fall off the rifle - oh yes, it happens...);
- iron sights actually regulated with test target (sights on many DG rifles are purely cosmetic);
- appropriate barrel contour and weight (recoil will not beat you to pulp);
- available used-as-new in "traditional" .375 H&H or .416 Rigby starting in the $900 range;
- commonality of fit/handling/feel with your .458 Lott (this alone is an overwhelming reason).
Admittedly for a .375 H&H chambering a true magnum length action is not a requirement, but it gives a lot of flexibility regarding bullet choice, seating depth and cartridge length, and everything else applies: Zastava (under whatever name, as well as other Mauser clones, e.g. Santa Barbara) and Winchester 70:
- have no integral scope bases;
- have unregulated screwed-on iron sights;
- have a magazine capacity of only 3 rounds;
- have a thinner barrel.
In addition, most Winchester 70 generally have a "pot metal" bottom metal, although this depends on when they were manufactured..
These are real considerations for me since I have had a "pot metal" bottom break at the hinge (on a .340 Weatherby Mk V) and I have seen (admittedly on harder recoiling rifles: .416 Taylor and .458 Lott) screwed-on scope bases come loose after a few hundred shots, and a screwed-on front sight fly off the barrel - both installed by a very well know custom shop...
For both CZ 550 and Zastava, I highly recommend installing a bolt-mounted, firing pin-blocking safety, i.e. a Win 70 safety. The "improved" action-mounted, sear-blocking safety is vastly less reliable as in a hard fall the firing pin cocking piece can jump the sear. It has happened...
From my perspective, using the "same" rifle in .375 H&H and .458 Lott is a HUGE plus when it comes to cross-training and familiarity. I personally went all the way and had a .300 Wby built on the CZ 550 magnum action for PG, so I can use my .300 Wby, .375 H&H and .416 Rigby with the same muscle memory and no conscious thinking required. As stated above, for this reason alone I would suggest you stick to CZ 550
(See
https://www.africahunting.com/threads/1st-traditional-dangerous-game-rifle.59160/page-4#post-701080 for further discussion).
