Mate, the model name means nothing. A 1390 could be named as 1234 by it's internal programming and it would still function as a 1390. Those other area units such as you mentioned are 1390s with a different model number, that's all. In some places, maybe they did have a mike fitted in the case but again it means nothing without the BT module. The thing which determines the BT function is what hardware is included to enable BT, and my US 1300s doesn't have the BT module or the mike. Have a look at this video, it's a US 1300:



See when he finally opens it, no mike lead to the PCB same as my US 1300:

1300 open

If you want to check, open your unit following that vid to have a look. If you do, be very careful when finally splitting the case so you don't pull the screen ribbon off the board accidentally.