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1) Yes it matters. The L1 & L2 labeled wires are the hot circuit feed from the electrical panel (L1 & L2 = LINE) and terminates to the L terminations.
2) The T1 & T2 labeling is the LOAD side wire which extends out to the connected LOAD.
3) Tstats are commonly labeled as L & T.
L= Line voltage source
T = Thermostat or connected to the LOAD.
4) If you have either a 2 wire lead AC voltage tester (a contact type) or better yet an AC voltmeter, the LINE side wires can be confirmed by the use of voltage tester to confirm the presence of constant voltage.
5) Since you have 2 black house wires extending to the Tstat wall box, sounds like you have a 240 volt supply?
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