It is known in the industry as a Linn type mounting. Distance from the platter spindle however might not be correct for all arms designed with the Linn mounting pattern. If the effective length of an arm is the same as the P6 then you will be OK.
That is a pretty good tonearm really. If your having difficulties with it there is a PT6 for sale here on audiogon. You might also consider the newer PT9 from audioquest. If your looking to upgrade the quality have you upgraded your cables yet? That would make a huge sonic improvement with your current arm and table. As to your original question, I don't know what else will fit the same mount. Sorry.
I had a PT6 for some time. Change out the cable before you go anywhere else. That makes a big difference, for sure.
I recently upgraded for a Moerch DP6. Phew - no comparison there in performance. Amazing arem, that Moerch, but I did have some mounting issues to contend with.
The PT6 has a rather large mounting hole ... the DP6 (I must like the #6) rather small. I had to route out a flange in the plinth, top and bottom, then use 2 steel washers to mount the Moerch. This gave me a little play in the overhang distance, which is quite similar to the AQ, give or take a few MM. The Moerch PT4 has the same mount as the DP6, I believe.
I was unable to find another higher quality arm that was closer to the mounting distance than the Moerch. Perhaps the Hadcock, but then there would have been cartridge compliance issues to contend with (as there are with all unipivots).