SME 309 HTA Key. Does anybody know how to get one or an alternative?

I just checked my box, & it's a torque wrench. 

HTA key is similar to a T6 torx.

A Torx head is not the same as a hex head.  A hex screw has a six sided depression, while the Torx depression has a 6-sided star shape.

From the instruction manual danmar posted, you are looking for a common allen hex driver.  Get a metric set with enough small sizes and you should find one that fits perfectly.  If you want to get fancy, you can get a ball-ended hex wrench set.  It is this ball-ended type that SME apparently supplies with their tonearms.  My Basis Vector arm came with ball-ended hex wrenches too.  They essentially work the same as regular hex wrenches, but they are easier to insert because you can come in at an angle and they will still fit. 

Sorry, but the SME HTA key is still a T6 torx.

refer to page 14 (Step 421):
https://sme.co.uk/audio/wp-content/uploads/sites/1/2016/12/SME-series-iv-instructions.pdf