I finally got myself a SBBN015 quartz tuna and took some comparison shots of the tuna can and the Fieldmaster, SBDC011 that I bought last year.
I measured the diameter of both watch from the back with a ruler and found the difference to be roughly 0.5mm. The Field master measures 48mm while the Tuna is at 47.5mm.
Below are some pictures showing the difference in the shroud, lug and crown design.
Lastly, here’s the stealth Fieldmaster, SBDC013 on nato strap.