The speedometer cluster it's self are completely different internally, if you want this to work correctly and read accurately you will have to have a 150 speedometer cluster or the guts from a 150 cluster,
I asked a reputable instrument rebuild shop about this several years ago and got the facts straight from them , if you could change over 120s to 150s, 150s wouldn't be as desirable as they are.
I sold one a couple weeks ago with a 150 lens and think I got two more in storage. I think they have the 15 lens on those, I will double check on this.
The 120 lenses can't be converted to 150 or 15 lenses, they are what they are, Sorry.
They also made two different lenses for these.
A 150 lens and a 15 lens, the 150 was used on early and mid 68s, while some of the late 68s and all 69s used 15.
The 150 lens has way too many ZEROs and looks like a solid row of numbers. Confusing, and you had to look at it too long to determine your speed. I think the Chrysler engineers figured this out after the production ball got to rolling and made the mid year change.
Come On ! , I know some of you Guys remember the "Pain in the Butt" 150 lenses.
