Sounds like they're the GHS Boomers. The funny thing is they're one of the stiffest rounds around. Nothing like the Sunbeams.
Or, they could be the Round Core Boomers, which are still stiffer than the Sunbeams.
I actually have GHS Boomers on another bass I'm trying to sell and they do look identical but don't really feel similar.
Either they improve a lot with age or they are round core.