On mine, which I am pretty sure is an original, and the few others that I have seen the markings were all on the outside of the glass. I have never seen an original with internal markings. Mine were obviously painted on with a small brush before I bought the plane in 1968. I have added them outside the glass to other straight 108s using the decal kit sold by Spruce and others. You probably don't have any internal markings, red lines, (U and N) yellow, etc? The other reason I think mine is an original the airspeed numbers are all florescent, if you shine a bright light on it they glow for a little bit. I don't have a radiation detector to see if it is a radium containing instrument.
My plane was N97584 when I got it but I changed it to N584LW which retained the serial number and added my initials.
Interestingly, the FAA later assigned N97584 to a Cessna which later crashed and there was fatalities. Then, they later again assigned it to a Cessna twin, which crashed with fatalities. If I was supersticious, I ought to be very happy I changed the number! Strangely, I can't find these in the NTSB data base now, but Have seen them previously.