The small square symbols are just a generic symbol for a transducer. Unfortunately, in this form there really isn't much of a standard, at least that I am aware of.
Generically, a transducer is simply a device that changes one physical quantity to another. Most often one of the physical quantities is electrical, although that isn't a requirement. Since there isn't much of a standard for these the language they were drawn in probably has a big effect on what the quantities are, since the letters are often the lead initial in the quantity being referenced.
For example, with the S/U case, you are probably correct that the U is voltage and the S is likely position. As James Mcquade said it is likely an LVDT but it could be any technology that converts position to voltage. It is also very likely that it is a "Feedback" transducer; that is, the physical quantity determines the electrical quantity. This is as opposed to a "driver" transducer, which would convert an electrical quantity into a physical quantity.
This is a completely wild guess, but it is possible that the orientation of the U is being used to indicate the direction of conversion. So in the S/U case it is a feedback transducer and in the U/G case it is a driver transducer. In that case the U/G things are just valve drivers. However, it is just as likely that the drawer mirrored the block and didn't correct the text.
I suggest you look at the blocks with the general idea of generic transducers and in the context of the larger schematic they are shown in. It may help you figure out what they are supposed to be.
Keith
