You can always remove the "extra bit" from a standard Nato to convert it to single pass through style.
Then, with a dab or two of glue and couple of stitches, the bit you removed can be made into a matching accessory for the combo. ;-)
It's also easy to further alter your now pass through style Nato to the Haveston sliding keeper style. I use two pairs of pliers, jaws taped to avoid marking the metal. Gripping the sides of the keeper second from the buckle, as far down as as you can, carefully pull and spread the join in the keeper, which is hidden in it's stitched sleeve. Just do a little at a time until you have spread it enough to be able to remove it. Close the gap and refit. You need to take care when removing the strap from the watch as it can slide off and be lost. If it moves when being worn press the ends of the join in, and this will tighten the gap. The whole process can be reversed, although this is a little more difficult.
Before and after.
Fastened.
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