DIODENMANN OF THE VINTAGE COMPUTER FORUMS WROTE:

I recently got a NOS bunch of these controllers. Instead of GDC-66 it reads TN-860C on the back but it's the same card.
According to the original manual, I can provide some information:

+-----------------------------------------------+
| JP1:                                          |
|                                               |
| PC or PS/2 reduce write current selector      |
|                                               |
| 1-2 : PC/XT or AT                             |
| 2-3 : PS/2                                    |
|                                               |
+-----------------------------------------------+
| SW1:                                          |
|                                               |
|  1   2   3   4                                |
| on  off on  on  : XT                          |
| on  off on  on  : AT with HDC installed       |
| off off on  on  : AT without HDC              |
| on  off off off : PS/2                        |
|                                               |
+-----------------------------------------------+


The manual also mentions explicit support of 360k, 720k, 1.2M and 1.44M drive types.