The Lyman 48 sight should fit precisely the contour of the mating receiver bridge surface to which it attaches. Unless the receiver bridge or the sight base has been altered, presumably it is not the correct sight; such particularly so when the sight is not a mix & match situation as you confirm.
If the base fits securely, it may still work but also the graduation markings may well be incorrect. Normally all but the earliest of these sights bear a stamped model designation code. Please be sure that you have examined it thoroughly. From recollection, I believe the 'hump' to which you refer is proper for the Springfield rifle and the 48 for the Model 30 is without that feature. These Lyman sights came in three differing sub series representing differing eras, and also each featured differing slide lengths. The longer slides became far less popular in later production eras of this sight.
In conclusion, by all accounts you simply have the wrong Model 48 sight for your rifle. All Lyman 48 sights in good condition are valuable. So you can likely find a correct one and market the present one. However, the value will be far greater if you can establish what rifle it properly fits.
Good luck!