Below is a bulb type pressure tester for testing the carburetor needle seat valve as referenced in TSB #564.
It's for use in testing the seat valve of Tillotson carburetors used on 1966-later Sportsters and 1967-later Electra Glides.
Click Here to go to the TSB page, scroll down and click on the “Y” next to TSB #564 to download the bulletin.
The bulletin was an attempt by the MoCo to diagnose a simple needle/seat issue and replace if needed at the dealer level.
(instead of sending the carb back to the factory for a warranty claim)

This bulb tester is HD (94750-68).
Per the service bulletin. it's installed on the fuel inlet fitting with the vent plugged.
Squeezing the bulb adds pressure to the inlet and the gauge shows how much is added.
A moistened needle and seat should hold app. 1 to 1·1/2 psi and release at approximately 3 to 5 psi.
A dry needle and seat will not hold as well as a moist one.
The gauge is for low pressure (15psi max) which shows a more accurate reading than higher pressure gauges.

Here is the bulb tester. Click on any pic to enlarge:

HD Needle-Seat Bulb-Type Leak Tester (94750-68) 1)