It really depends on what u define as 'compatible'. If it is just whether water can flow out, then yes.

But if what u meant by 'compatible' is to have the same water flow / water pressure as your old National shower handset and hose, and can fit nicely with your old National handset holder, then I think even Panasonic salespersons dare not guarantee it will be 'compatible', since your National instant water heater has been discontinued for production more than a decade ago. U will have to take your own risk.
U should also buy a new handset holder too to match the new hose + shower handset (same brand / set), so that the hose + shower handset can fit nicely on the handset holder. And do expect water flow / pressure to be different than your old National shower handset, since it depends a lot on the handset's nozzle size and quantity. The smaller the nozzle size, the higher the water pressure (sharper and further water spray); the higher the nozzle quantity, the higher the water flow (provided water pressure is sufficient).
Also, make sure the new hose + shower handset you are buying are non-conductive, as per Suruhanjaya Tenaga guideline.