I've not got them on my M&P 9mm yet, but have the XS Sights Big Dots on my G27. Both front and rear have tritium.
If I recall, due to the rear sight dimensions, XS was unable to put a tritium ampule in the rear site for the M&P. So they'd have the a Big Dot (or Standard Dot) with tritium and an express rear sight without tritium.
I'm not 100% on this though so please don't take this as gospel. Check with them to confirm................
skeleter is correct. I had Dan install the XS Big Dot Express sights. This set is tritium front and no tritium rear. They claim they cannot get a tritium insert in the back sight because there is not enough room. This is why there is not a 24/7 set for the MP.
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