Greetings...myself being a 'Pebble of the Community', I would defer to the folks comments above.
I just wanted to point that on Variety Vista web site, under "19th Century & Misc.", there are about five or so Liberty Seated dimes
with what looks like doubled 'S' Mint Marks. So they exist. Or called Overlapped.
My take is this was definitely a double or triple 'S', and as noted as a possibility by the Bedrocks, got mangled over time, and hard to officially determine. But my money is on a once Double/Triple/overlapped MM/RPM, and you now see what you see on yours. In fact, yours looks like it may have been a dramatic overlap. Mangled, but a lot of 'S' there IMHO.
Just as a side note...Variety Vista, on all of these samples they have posted, the source is "unknown". They are asking for physical samples to be submitted for Attribution and photography ("Need Specimen for Study"). You may consider that, unless it is too damaged to be of use. Don't hurt to ask. They will tell you.
I had an email correspondence with Variety Vista's owner, Dr. Wiles, on a different coin matter last year, and he is a very pleasant individual. Very helpful. Very committed to his craft. Up to you.
Here's a couple sample pictures from V V: