[sdiy] Question: Vactrol equivalent part numbers

[RMC]

My Korg PS-3100 has some LDRs in it - three are "HTVP873D" and another three 
are "HTVP873-G35-201B" and I want to buy some spares.

The first type contains two LDRs with a common terminal, the second type is 
a standard single LDR.  I have searched and visited several websites but, 
short of demounting the LDRs and measuring them, I don't know their 
characteristics and can't see what equivalents there might be. the Perkin 
Elmer site didn't throw up any equivalents.

Does anyone know these devices, or can anyone tell me model numbers for some 
similar ones?

Cheers

RMC, England