@ChristianJ: German Post is in the process of switching from clichee machine stamps to inkjet machine stamps. There were tests on real mail as early as 2018 at least (e.g. in Briefzentrum 60, Frankfurt am Main), and some letter ceters have already completely switched over to them. The new inkjet stamps look the same as the inkjet stamps already in use for large letters (Großbrief and Maxibrief) for many years, in some cases they even use the same ID code (e.g. md in case of Briefzentrum 60). You can distinguish the Großbrief and regular letter cancellation machines if you have the complete cancellation (e.g. the complete envelope): Großbrief cancellation is always only the actual cancellation information (Briefzentrum with number, date etc.), while the regular mail size inkjet cancellation always includes an advertisement area (or the wavy lines as in @ChristianJ’s initial German postmark). I’ll show pictures early next week when I’m back home and can dig through my collection.
Some additions to the great informaton provided by @Oo_Hawkwind_oO:
- The letter center number equals the first two digits of the postal code (ZIP code) except for a few large letter centers serving two regions (noted in the map by lines to both regions), and since the end of 2003 for the region 42: When it was closed, its region was divided between letter centers 40, 45 and 58.
- The time © in the cancellation is not the actual time of cancellation but the time of the end of the processing step. In most cases all mail accumulated during a day gets a time of -20, -21 or -22 (depending how the individual letter center is connected to street and air transportation) regardsless when it is cancelled. Some letter centers have some late-service postboxes (for Briefzentrum 60 they are in Frankfurt Hauptbahnhof (central train station) and directly in front of the letter center) where late mail will still reach recipients in the letter center’s area on the next day, but all other areas the day after, such mail can be identified by a later time, in most cases -24 (for midnight, even though the actual processing might be later in the night).
- The ID code (Unterscheidungsbuchstaben in German) are uniqie per cancellation machine, but each cancellation machine has two stamps (so letters/postcards can be cancelled facing either side, thus saving a step to get all stamps facing the same direction). Sometimes the advertisement part of the cancellation for the two stamps of one machine are different (they are sold individually to those who want to advertise), so you may get two different advertisements with the same date and ID code.
- Germany still has localized cancellation (i.e. cancellation stamps showing the name of the city). If you go to a post office and ask nicely, they will cancel your mail by hand with something saying e.g. “Frankfurt (Main) 503” instead of the Briefzentrum and witzh the postal code instead of the postal horn logo, in the example 503 is the internal post office number of that city. Small towns with only one post office may not show a number, and more recently postal outlets in shops have other stamps, e.g. Frankfurt-Preungesheim, as these are not post offices (the clearks there are not from German Post but from the partnering shop).
@sannah82: Correct. I haven’t yet found a comprehensive list of which letter center does which regions. They don’t seem to be by numbers - 35 Kassel goes to 60 Frankfurt, not 30 Hannover. There are roughly 10 letter centers working on Sundays. I’m certain about 30, 60, 80 and 90. Sunday cancellations are easy to spot by the hour ©: It’s between -12 to -16, not in the 20s as for workdays.
@aerobear, your energy awareness month cancellation is not considered a commemorative cancellation, USPS is either using the advertising space themselves or has sold it for this. Same for the Happy Holidays cancellations each year. The USPS did have commemorative machine cancellations in the past, e.g. for first day covers, I’m not sure if they still do (haven’t seen any recent first day covers from the US).
Sorry for nitpicking - I’m a philatelist specialized on postal automation, so cancellations, especially by machine, is part of my core philatelic collection.