Hey Jdrom,
Yes I believe they have different, not sure what exactly is different but the ACK-I7751 is for the 3 heat pipe heatsinks and the ACK-17753 is for the 4 heatpipe heatsinks..
Anitec has the 4 heatpipe one in stock..
Anitec.ca - Xigmatek Crossbow (ACK-I7753)
Maybe it could be that the brackets provided are slightly adjusted or something since the 4 heatpipe covers a little more processor space, hence probably stretches out a little but on both sides..
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