All 6155's made until July 1 2012 were for 27mm, we changed it to 20mm as the new 6255 is the better fit for most tanks that are large enough to have 27mm or 1" glass. 6255 will have the bigger 27mm magnets now. Basically, if it is out there now, it has the 27mm magnet, the new ones that are being produced now, will have the weaker magnet.