Q:

How to ground a vintage device that was powered without a ground?

This is a follow-up to Is there a mathematical relationship between the battery AH rating, and that of the transformer in the charger 
Since that post I've uncovered a vintage battery charger (Manufacturer: Philips Model: PK-5000) in the old house itself. The device looks good, albeit it has probably not seen service in easily 3 decades ... or so; certainly not since I was a little boy.
It has a 2-point supply (without a ground point); what should be done to ground the device so it may be powered by a proper 3-point plug?

A:

Nothing. AC doesn't need ground to function if it has neutral line. You can just put a 3 pin plug and not connect anything to ground pin if you must use a 3 pin plug. 
If you want to implement the ground yourself for say additional safety, post pictures of the insides of the charger (In fact post pictures of the insides in any case! We may identify some component that may have failed over time with them). Most such devices I've seen just have the ground wire connected to the case of the device.
If you find electrolytic capacitors somewhere inside, do change them! They tend to wear out with time. 

A:

If this charger has metal case, then you can connect ground wire to the case. However, first measure that there is no connectivity between case and any line right now. If there is connectivity, then do not connect ground and do not use it at all. 

A:

If a device has exposed terminals and is supposed to be 'free floating' relative to earth ground, but you have doubts about its electrical safety, perhaps you should wire a GFCI/RCD into the plug?  That should avoid the possibility of a lethal shock hazard in the event that something becomes live that isn't supposed to.

