The Silplug from the mid 1960's is a plug-in replacement for the 544 type of HT rectifier. The inside is obscured but as the name suggests it will have silicon rectifier diodes as the main active components.
Testing the continuity between the heater pins indicates that there is no replacement for the heater, thus this is simply a pair of HT rectifiers.
The original 5Z4 was designed to supply equipment of up to 40 Watts total power and was made by several manufacturers.
The makers details.
Scratched into the top is 500V whereas the 5Z4 has an anode rating of 350 Volts.
The IO base. Note the hollow spigot - no glass seal to protect.
From Short Wave Magazine in June 1966.
The Paxolin metal tube envelope is 32 mm in diameter and excluding the IO base pins, is 60 mm tall.
