It's definitely a skull. If you shrink the sh1t quality photo you can make it out.
Probably Long John Silver's Keelhaul Squadron. Load of T-shirts on sale with that insignia on so not military unless the SS stands for space shuttle.
Then there's the little gay mincer version
Why don't you write to Howard Burden who did the costume design for RD and ask (nicely) if he would elaborate where he found it/what it is. Perhaps he may even have a clear photo of it. Just ask very very nicely as he may be a very busy person.
And please don't ask me how to contact him, it's pretty easy to find contact details for him on t'interwebs.