Question about air tank gauges

After complaining about the difficulty with the pressure relief valve on my apeks tank valve for months I ordered and received a valve from pyramid air. This unit has a screw valve release which should be much easier for me to operate. NOW a question. The glycerin filled gauge is much larger on the apeks valve and I'd prefer to have that on the new valve. Looks like it should be a simple screw off one and on to the the other deal. However there appears to be a drift pin on the one valve which concerns me.

Anyone know its function and can I safely unscrew the gauge with out damage to anything or should the the pin be driven out first. Any thoughts or should I just leave well enough alone. The apeks valve is probably of no value to anyone as it is so if I can use that gauge even it it's of only slight advantage why not. BUT I don't want to screw the pooch either.
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Yes it sure looks like a drift pin it goes threw to the other side of the valve body or there is another (what ever it is) on the other side. I'm all but certain it's a pin to retain something internal to the valve or is there to prevent that threaded fitting from being backed out.

i just don't want to attempt backing that fitting connecting the valve and the gauge out without knowing for sure what I'm doing. 

Clearly I could attempt to tap the pin or unscrew the fitting but I think I need to know what's what before I do ( and bugger something up)