This is what I can gather from the files on my unit (2545) after doing updates (unit bought in the first half of this year):

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Can't tell about 2014.30 because I don't have a 2545LM and I can't do more updates directly from Garmin, also I don't know if devices from other series and different map coverage get a different treatment but I suppose it should be relatively similar.

As you see the D*.db files do change between updates (different md5sum) and the system.db file gets updated accordingly, that is why I said before that it is not clear to me how to update D*.db and test if it is working properly, or if it is needed at all if placing all the needed files in the sd card. As far as I know the "recommended" update method is to leave the files in the unit alone and place all updates in a sd card, I suppose this situation is similar to buying a map with a different coverage and installing it to an sd card.

In the update from 2014.10 to 2014.20 it seems that not much has changed. At first glance only the filename, dates and the last part of what it seems to be the part number have changed, I don't know if anything else has changed as I didn't compare that. Still a D*.db file with some (minor) changes was downloaded and the system.db file was updated accordingly.