You Are looking at the Yamitsu / Kockney Koi Tank Liner Connector
1.5" - 43mm flange size 114mm
2" - 56mm flange size 114mm
3" - 82mm flange size 154mm
4" - 110mm flange size 198mm
These will require the use of a sealant around the flange to form
a good seal.
PLEASE NOTE WHEN STOCK HAS GONE THAT WILL BE THE LAST