There is no level shifting as Sin is not used, you are transmitting using the stamp power.
Check the tx pin directly first (with the scope), so you know whether normal or inverted is needed (maybe firmware is smart, or maybe not). If normal looks correct, you need inverted, if inverted looks correct, you need normal. If you can't decode output from tx, then anything else further down the circuit will not matter.
It looks like Sout will be pulled down to Vss through a pair of 10k's in the npn pack, then 4.7k to the Sout pin- to make it stronger tie Sin to Vss.
OK I am an old idiot
You are using a Basic Stamp, you may be old. You can't be an idiot if you found this forum.