The dryer is built into the receiver, and sits in the liquid line between the condenser and expansion valve. It was probably replace if o-rings were changed, because that opens the system to air. Additionally, the desiccant used in the old R12 dryer is not compatible with R134a and its oils, so it should always be changed for a conversion.

Also, there is a low-pressure cutout switch on the dryer, which should be replaced with a binary (high-low) pressure switch. Any replacement switch will be a binary switch, since it never hurts to use one for R12.

If you have a restriction in the dryer, the inlet line will be hot and the outlet line will be quite cold. The same goes for the condenser (or any other liquid-bearing part).