Civil War McClellan saddle stirrups were made without the later stamped US in an oval on the leather hood itself (similar to the CW Union belt buckle) but this occurred during the Indian Wars time period in the late 1870's and was in use from that point on until the McClellan saddle was done away with before WWII. That stamped US in an oval is correct for those later time period stirrups as shown by ucvrelics and would have been on thousands of these stirrup "hoods." After the ACW, the width of the wood stirrup tread was increased to 4" wide about 1869.
I have no idea why the US brass disks from horse blinders were applied to those stirrup hoods and why they are missing the stamped version on the leather.