Normally, you will get/buy a pair of radiator vaves, one will have a plastic wheel head with which you can open and shut as required, the other (return end, recommende) will also have a plastic wheel head but this is only a cover and turning it has no effect, you just pull this up and open and shut this valve with a adjustable spanner, if your system is balanced then this valve will be set to 1/16,1/4,/1/2, or whatever open and don't touch this again, hence the lockshield name, you just the replace the plastic cap and if you wish to shut off the rad you just shut the the other one, when its reopened then the rad is still balanced.
You said
"At the larger value side (thermostatic) I measured a temperature of 54.2 C and at the smaller value side a temperature of 56.7 C. There were sometimes variations, but generally it's about 2.5 C colder on the thermostatic valve side "
This means to me that you have a thermostatic radiator valve (TRV) installed on the return and a wheelhead valve at the flow end, normally, it is recommended that the TRV is fitted on the flow side and the wheelhead on the return, they can be installed as yours appear to be but then the TRV must be of the bi directional type, ie, two arrows on the valve body, some installers here say that even if bi directional that they should always be installed on the flow side, mine are all on the return.
Can you post a pic of both to make sure we are talking about the same thing?
You said
"At the larger value side (thermostatic) I measured a temperature of 54.2 C and at the smaller value side a temperature of 56.7 C. There were sometimes variations, but generally it's about 2.5 C colder on the thermostatic valve side "
This means to me that you have a thermostatic radiator valve (TRV) installed on the return and a wheelhead valve at the flow end, normally, it is recommended that the TRV is fitted on the flow side and the wheelhead on the return, they can be installed as yours appear to be but then the TRV must be of the bi directional type, ie, two arrows on the valve body, some installers here say that even if bi directional that they should always be installed on the flow side, mine are all on the return.
Can you post a pic of both to make sure we are talking about the same thing?