Usually stamped on the outside of the handler.
trane air handler tonnage for model BWV760P100A1
3 tons
3 ton
TM2005 06905
Inside unit that distrubute air threw a handler unit
1.5 ton
Two and a half ton
1.5 ton
The air handler does not have a tonnage rating, the evaporator coil within it is what you`re looking for since any given air handler can accomodate several different coil sizes. For the answer to your tonnage question see the tag on the coil, though that may prove to be difficult since it is enclosed in the plenum. An easier method would be to get the tonnage from the condenser model # in the yard.
Two and a half ton
The B19 is the model or series. The 51 is the tonnage( the unit is actually a four ton unit with the coil being a little bigger for increased efficiency.
While i do not know the answer to your question, i can say if you have a chrysler air handler it is definitely advised to get it replaced. They stopped being made around 40 years ago, and parts are no longer sold for them from the manufacturer.