I would take Smith Enterprises at their word. They are know for quality products. I've never had one of those USCG flash suppressors, but they look to be an excellent bolt on product. You should be able to simply reuse your M305 castle nut and mount it up. I know a USGI flash suppressor fits perfectly on on my Norinco's, using the Norinco castle nut of course.
WE are hoping to get these in asap. It was backordered with SEI for over 2 months already and they are in production.