Who we are not.
SNPs are the best and only unambiguous way to define branches of the Y-tree. There are a lot of STR clusters that look very similar to ours in terms of their haplotypes. It's often necessary to test more markers to determine for certain who your closest y-line relatives are.
Similar but Unrelated Clusters
Depending on how many markers you test, you make look like you belong to any number of groups. The ones most commonly mixed up with the LSC are the Scots Cluster, Irish Type III, a DYS391=11, DYS464a=13 group, and the Bentley/Drake group.
When fewer than 67 markers are tested, there are other groups whose member's haplotypes are very similar to our own, in particular, Irish Type III. Our most striking feature at the 25 and 37 marker level, that of having 13-15-X-17 for the DYS 464 markers is also shared by them, where it is almost always 13-13-15-17. They however tend to have DYS 459 values of 8-9, where we have the typical R1b values of 9-10 (although several members have 9-9). They also tend to have DYS 391 equal to 11, rather than our characteristic value of 10.
Similar and Possibly Related Clusters
One cluster which could be confused with the LSC if only the value of DYS590=9 was search for would be the 21-5909-A cluster. There is no special project for this cluster, but you can find its members in group K on the R-DF21 FTDNA Project. Men in this cluster have tested positive for the SNP L1336.
The only off-modal characteristic marker this group and the LSC have in common is DYS590=9. However, it is possible that the DYS590 mutation may have happened just once, and that both clusters inherrited the value from their shared ancestor. Future SNP testing should reveal whether these two clusters are closely related or not.