Pretty high in the ranking for the oddest Mauser, and my favorite Mauser, the Spanish FR 8, they were made from the late 40's well into the 50's. It's a Spanish M43 8mm large ring Mauser reconfigured and rebarreled to 7.62 Cetme/Nato. In the very early days, the Cetme had issues with Nato ammo, so Spain made a lower power version ammo the 7.62 Cetme. After some revisions to the Cetme it became an excellent weapons system and the issue with the 7.62 Nato round was fixed. This was designed and built to be similar to the Spanish Cetme to assist troops with becoming accustomed with the sighting system, and was commonly issued to rear line troops. The Cetme triple ring, flash hider and front sight tool, bayonet and sling are shared with it. The barrel is also the same, just a modified Cetme barrel, designed for a Mauser action. The combination of the short 19 inch stiff design of the barrel, the storage tube and bedding in the stock add up to a very well bedded action, once you get one dialed in, they are pretty accurate. The tube under the barrel serves double duty, the bayonet latch and a removable storage tube, for cleaning kit, a nudie picture or a cigarette. The rear sight is a V-groove for 100 meters, rotating it changes it from 200, 300 and 400 meters, these distances being aperture sights, getting smaller and smaller apertures as distance increases.
You can aways just tell someone who is clueless, it's a gas operated bolt action, see the gas tube under the barrel.
There is another version, the FR 7, it differers from the FR 8 some, it is based off the M1916 small ring Mauser, and the biggest give away is the straight stock design vs. this ones later K98 style stock design. They are somewhat less common as well.
These first photos are from the seller I brought my first FR 8 off of many, many moons ago. That dude could take wonderful photos
This is one I unbubba'ed it's a 1947 dated FR 8. Handguards are like finding a, well ok, you can't find them
Here is my 1947, 1951 and a 1955.