7 digit odometer ?

[72 K20]

I recently bought a 71 Cheyenne C20 for parts and noticed today that the odometer has 7 digits , 6 black and the white one for tenths. I looked at my 72 C20 and 72 K20 and a couple of other clusters and they only have 6.

Everything else on the speedo looks normal,,, but there are 7 digits.
I've owned 7 of these trucks and have never seen this before.
I just went back out and checked again to make sure that my eye's weren't playing tricks on me and there are 7 digits.

Was this an option ? The bottom of the speedo face plate appears to be original ( not cut out ). Maybe an after market speedometer ?

I guess my wife has the good camera with her, so , no pics tonite but definately tomorrow.