levers,
The upper part of the Spencer "sear" arm, (the part the trigger works on) has a radius cut into it. This is because the "outer magazine tube" will not allow the sear to move up high enough to function correctly.
Look to see if your sear has this radius? If it does not, then you have a "Sharps" sear in your lock.
I hope this helps.
My best,
Blair