HOWEVER, as stated earlier in this thread....In the US ultralights aren't aircraft; they don't have to comply with any regulations other than CFR Part 103. Part 103 says nothing about markings....if there is a regulation in the rest of the CFRs about markings (there is) it doesn't apply to ultralights. No matter how bad a day the FAA inspector is having he still has to cite a regulation for enforcement. If your fear of "the man" is such that you are unwilling to follow the regs as written then that's another problem.
But that is not something that can be interpreted as an illegitimate registration number, CG.