It does look like Signet in many respects. Hull shape, transom hung rudder and very shallow draught. If it is then it is very much older than the only picture I have, ca 1980, or the one based in the yard I use.
I'm still not certain that the boat on eBay is a Signet because as far as i can discover they were either fin or twin keelers. I've not found any mention of triple keels but there were a number of different builders so maybe someone did a triple keel version but heavens knows why.
As the vendor knows little about boats I did not take the overall length he quotes to be necessarily accurate. In my many reference books the Signet is shown as not having a transom hung rudder, and the one I nearly bought in 1980 did not have a transom hung rudder.