Here is a post I lifted off a thread here called "Wyandotte combs?"
Speckledhen is right, the single comb comes from there cochin ancestry, along with other breeds. Breeders keep a the single comb gene floating around to increase fertility. For some reason fertility is low in rose combed birds and this can be improved with the addition of single combs into the gene pool. Single combed wyandottes cannot be shown but they can still be considered normal and can be an advantageous addition to a wyandotte breeding flock as they can still produce rose combed offspring.
If you do a search here in Breeds, Genetics and Showing for SLW combs, you will find lots of discussion of this. I THINK the roos with single combs are more likely to be fertile. Mine are 11 weeks, rose combed, and I can see that I have roos and pullets; will be interesting to see how well they reproduce.
So, yes, it is probably a SLW.