As you probably already know, that's not actually an antique, and it was not made in 1845.
It may have originally been a Spanish M43 Mauser, which simply had the receiver ring scrubbed and then the markings crudely restamped, but with the date changed to one in the 1800s to get around the bringback restrictions. Or, it could be some sort of other Mauser 98 action, cobbled together with various Mauser parts to make a complete rifle, and merely stamped using a Spanish M43 as inspiration. Either way, they didn't make those in the mid-1800s...
You see quite a few of these fake "antique" rifles that servicemen have brought back. There's an entire local industry in Afghanistan of taking newer surplus rifles and making them into nominally pre-1899 rifles so that the American soldiers will be able to buy them and ship them home. Some of these conversions are as simple as overstamping the 19XX in a date to a 18XX, while others go to greater lengths to disguise the origin of the rifle. Either way, most of them are obviously not antiques to folks with a little rifle knowledge, but apparently it's enough to fool whoever approves the bringback paperwork!