Your chassis maker could tell you if you should have a dip stick and also likely has a PN for it. Call them with your VIN.
Wonder if the dip stick is missing or has just come loose from the cap and slid down in the tube?
Can you use forceps or needle nose pliers to check for the dip stick?
OR (at least on the Allison 3000-- don't know which Allison 6 speed you have) worse case is you would buy the O ring/gasket where the dip stick tube goes into the transmission-- lift it enough to shove the dip stick up the tube. Fluid level is lower with the engine running.