Questions about gauges...

[69sixpackbee]

Are there any differences between the gauges from a '70 and a '72? I have a set I just pulled from a 70 GMC and I did notice the oil pressure gauges say what pressure they are, i.e, 60 psi & 80 psi . Are the gas gauges any different? I cannot remember if my '72 Chevy gas gauge needle was "parked" at the latest reading when I turned the key off. The GMC gauge, however, seems to go back to "empty" when I turn off the key. It does come right back to where it was as soon as I turn the key back on. Is this right? Are there different gas gauges? Also, is there any difference between a speedo for a 4-speed and a speedo for an auto?
Even though they look identical with the exception of the "hole" for the hi-beam indicator (the '70 has a slot and my '72 has a round hole) there are different part numbers for the two but they seem to function the same. Is the difference just the hi-beam "hole"?
Thanx!