You peaked my interest. looking through the unicel book,the only ones that might be close are called the. TC300
(tri-clops) It says there are 3ea, 100 sq ft filters. OEM part# 42-3640-10-R.
Then there is the TC450 (tri-clops) 3 ea,150 sqft filters, part#42-3640-15-R.
The last being TC600 (tri-clops) 3 ea,200 sqft filters, part# 42-3640-20-R
It does not state whether they fit inside each other or not? And there is no Unicel after market part # ??
Best bet would be to call Unicel see what they say. 818- 678-0400
Good luck!!
Let me know what ya find out!!