The length between the center of the two bolts is referred to as the center line.  The distance between the center of the tank holes must be within 1/32” of the center line measurement.
Items
B3540-11300BA1W

Standard Aluminum Liquid Level Gage, 13” Centerline, Front Mount 1/2"-20

B3540-11300BB1W

Standard Aluminum Liquid Level Gage, 13” Centerline, Back Mount 1/2"-20 with Nuts

B3540-11300BA2W

Standard Aluminum Liquid Level Gage, 13” Centerline, Front Mount 1/2"-13

B3540-11300BA1W

Standard Aluminum Liquid Level Gage, 13” Centerline, Front Mount M12 X 1.75

B3540-11300BN1W

Standard Aluminum Liquid Level Gage, 13” Centerline, 1/4" Male NPT Adapters
Centerline N/A 13"
Mounting N/A 1/2"-20 N/A 1/2"-20 with Nuts N/A 1/2"-13 N/A M12 X 1.75 N/A 1/4" Male NPT Adapters
Feature N/A Standard
Max Pressure N/A 175 PSI
Max Temperature N/A 225° F
Seals N/A Buna-N
Body Material N/A Aluminum