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The Cropico ESC1 was amongst the first Zener diode based replacements for the Weston cell.
Solid state voltage standards, such as this one, have the advantages of being more rugged (less sensitivity to physical shocks and electrical loading) and of having a much smaller temperature coefficient.
This one did not use a temperature-controlled reference diode but, nevertheless, the temperature compensation was quite effective.
It uses 18 (yes, 18!) C cells to power it, so battery replacement wasn't cheap. Cropico discouraged users from using an external power supply as they argued that (a) noise levels would be higher and (b) the thermal mass of all those cells added additional thermal stability. Nevertheless, many were successfully converted to operation using an external PSU.
This one is still in good working order.
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