Hi Steve,
I am not familiar with the specific light so can't speak to whether the adapter will work in it. As I understand, there may be quite a few variations of fulton lights out there that were used in the military (angle and straight versions, C and D size, perhaps others). If you can post or email or PM a couple pictures of the contacts in the head and tailcap that may be more revealing.
What sort of bulb does it take? PR flange?
As far as 6AA vs 2D. Generally speaking, 2D cells, if compared on equal chemistry terms, will have more stored energy than 6AA cells. The reasons that people convert to 6AA cells is either to get the higher operating voltage, or, in order to use specifically desirable cells only available on AA size or some combination of the above.
Examples: Someone who has a NIMH charger for AA cells, who does not want a bulky charger for D size cells, or doesn't want to buy the cells, may opt to convert, or someone who wants to drive a 6 cell bulb in a 2 cell host, or someone who wants to be able to use Energizer Lithium primary AA cells (not available in D size) in order to get longer shelf life may opt for a 6AA with 2S3P wiring configuration.
Eric