Simmons does not state where the Simmons 22 MAG 3-9x32mm Rimfire Rifle Scopes are made. I recommend contacting Simmons for that information.
The scope rings are dovetail mount.
The scope rings included with this scope are dovetail mount.
No, the Simmons 22 MAG 3-9x32mm Rimfire Rifle Scope includes 3/8 dovetail compatible scope rings. The Magpul Hunter X-22 Backpacker Optic Mount uses a picatinny rail. The Magpul mount is also not long enough to accommodate a scope.
You will need 1" low rings for the Simmons 22 MAG 3-9x32mm Rimfire Rifle Scopes.
The Simmons 22 MAG 3-9x32mm Rimfire Rifle Scopes w/ Rings will work with 3/8 dovetails.
The Simmons 22 MAG 3-9x32mm Rimfire Rifle Scope will include rings designed to mount to dovetail grooved receivers and should work for your Winchester model 290. If for some reason the rings do not fit your receiver, I'd recommend purchasing the Weaver Tip-Off scope rings part number - 49440.
The Simmons 22 MAG 3-9x32mm Rimfire Rifle Scope is designed for use on .22 Magnum Rimfire rifles. For a Marlin 336 in 30-30 I recommend using a scope like the one at the link below. You will also need a base or rings like the ones at the second link below.
https://www.opticsplanet.com/vortex-crossfire-ii-2-7x32-rifle-scope.html
https://www.opticsplanet.com/talley-lightweight-alloy-scope-mounts-for-anschutz-legendary-marlin-montana-winc.html?_iv_parent-gun-model=marlin-model-336&_iv_gun-model=marlin-model-336,marlin-model-1895&_iv_ring-height=medium&_iv_tube-diameter=1-in
The Simmons 22 MAG 3-9x32mm Rimfire Rifle Scopes w/ Rings might mount to that rifle but it is designed for 22 Magnum chambered rifles. I recommend using a different scope for your rifle chambered in 8mm Mauser.
The Simmons 22 MAG 3-9x32mm Rimfire Rifle Scopes parallax is set at 50 yards. This doesn't mean that the scope can't be used at a further distance it is just stating that there is no parallax adjustment with scope and that it is set at that distance.