This Savage manufactured No. 21 series micrometer rear sight is for the model 1905 bolt action rimfire rifle with a dovetail notch on the barrel for rear sight mounting and is in good condition exhibiting a dark brown patina with specks of rust present.  (file photo)

Note: Savage model 1905 bolt action rimfire rifles with the following serial numbers have been observed with this part: 3637, 7686, 11848, 26925 & 40282