Just to clarify,there's an observable difference between the US Optics MST-100 & the Unertl in that the USO has what's commonly known,a man hole cover at the bottom of the erector assembly.
This was an improvement by USO to replace the Unertl erector leaf spring with a coil.Some Unertl's that went through USO for overhaul have had this modification done.
By the way,I have a friend who has two of the last production run USO's for sale.