It's a design iteration (mark), a version number so to say, by Makerbot. MK6 nozzle looks like this and the MK8 like this.

Normally the threads on these these nozzles (MK6 and MK8) are a standard M6x1.0. But Like /u/iLLChosenName said, the thread is different on the heater block of your printer, namely M6*75.

If you are a handy DIY type of person, you could try to tap an M6x1 thread into the block. Due to the fine thread already inside, you don't need to drill and there is plenty of material inside to be able to tap it properly. But you need to be careful as not to tap too far so you don't tap the heatbreak threads. And be prepared to replace the block in case you screw up.

EDIT:Somebody actually already did exactly this: https://www.youtube.com/watch?v=A-ko64Pj3RE