For example a 1 1/2" steel case gauge with 1/8" NPT lower mount with 100 psi would be ordered as follows:

                        P  M   B   15   8   E    LM
    Type ---------------+  |   |    |   |   +---+ +-------Mounting
    P=Pressure             |   |    |   +----+  |          CB=Center Back
    C=Compound             |   |  Diameter   |  |          LM=Lower/Bottom
    V=Vacuum               |   |   15=1 1/2" |  |          FF=Front Flange
    T=Temperature          |   |   20=2"     |  |             (3 Hole)
    W=Water Test           |   |   25=2 1/2" |  |          PM=Panel Mount
  Case --------------------+   |   35=3 1/2" |  |             (U Clamp)
  A=Steel, Ammonia             |   40=4"     |  |          LB=Lower Back
  D=Steel, Diaphragm           |   45=4 1/2" |  |
  G=Stainless, Filled          |             |  +--- Range (PSI/kPa/Bar)
                               |      Connection      A=Vacuum
  M=Steel, Dry                 |        8=1/8"        B=15   I=400  Q=5000
  N=Plastic,Dry                |        4=1/4"        C=30   K=600  S=10000
  S=Stainless, Dry             |        2=1/2"        D=60   L=800  T=15000
  Internals & Socket ----------+                      E=100  M=1000 U=4000
  B=Brass                                             F=160  N=1500 V=6000
  I=Iron or Steel                                     G=200  O=2000
  S=Stainless Steel                                   H=300  P=3000