? - i would expect if anything, the patina and corrosion to be worse under the brass & roughly the same in the exposed holes, exposed to the same corrosive conditions. brass in contact with steel would set up a galvanic corrosion cell if not kept oiled. steel is higher in the galvanic series than brass and thus will corrode preferentially.
anyway, if trade blades were common at the end of the 18th c. it's possible an old patinated blade still in working condition may have been rehilted in the early 19th. as it appears to be shorter than the normal european blade, it may have broken off & the owner had it rehilted as a full exposed tang hanger/cutlass, thus the fullers extending under the filigreed brass. waste not want not
i suspect some bright spark thinking he'd increase the bling value has cleaned it & polished the brass recently. will know more when my supplier returns from france early this week 7 posts it to me so i can supply better photos.