It should be if I'm interpreting you vision of "flat".
Jon's are nice quality and offer better structural support for the SMA jack due to the way it is built and rests on the raised shoulder or collar around the SDS100's recessed SMA jack.
The base of the adapter is the same diameter as the collar around the SMA connector, with a slightly-tapered extension that press-fits into the collar when you tighten the adapter. So the collar supports the adapter from lateral impacts, reducing the probability of breaking the SMA connector.