In my database, this bus should be ex. Linjebus 6012. 6017 should be 329 APX. Are you sure? And do you have an email adress? I have other questions for you :)

Well, there are two options. Whether you have mistake in your database or there was some error on vehicle registration. According to my database, 329APX is ex Linjebuss 6023 and have chassis number 29035 and 748APV is ex Linjebuss 6017 with chassis number 28986. These data comes from lkf.ee site that gets their data from ARK.
Some years ago there were very popular to change number plates in ATKO group. The reason was that some buses did not have valid inspection, so they had to change plates to continue using technically bad conditioned buses.

It's a mess then!

I have now been in Tallinn, and I checked the bus myself, and you are right, this bus is ex. 6017. :)