DUH, he mentions PSI so we must assume it has PSI writte on it.
They are connected to the VACUUM side of the engine and consequently show the VACUUM or boost state ot play.
If you have never seen a boost guage iof the type talked about here you really need to get out more.
Are you even looking at the same as everyone else ?
I am not going to continue this waste of time with you, clearly you must be referring to another "LETTER"
If you want to get techy, they are incorect anyway even the MONOMETRES as they do not register the normal air pressure first!!,ie add 14 or so psi.
tabetha
ps what the hell are you reading ?