You can in fact determine the assembly number of the mainboard of a C64 without opening it. Here is how.

Some mainboards are in higher demand than others. Some are more error-prone than others. There are good reasons why you may need to know the assembly number. But sometimes you don’t have the option to simply open the machine and take a peak inside. This could happen when buying the machine from a remote seller or when selling a machine with an intact warranty seal.

Having been in that situation multiple times, I eventually learned that you in fact can can determine the assembly number with almost 100% accuracy by taking a look through the tape port. As you will see below, that exact area was redesigned with every new mainboard release.

In the table below, I explain what to look for in order to determine the motherboard revision for every single assembly.

Mainboard assembly number	What to look for
326298	This board only came in a breadbin case with dark keycaps. When you look at the tape connector, you see three ferrite beads.
KU-14194HB	This board only came in a breadbin case with dark keycaps. When you look through the cartridge port, you see a white coil housing to the left of the fuse. When you look at the tape connector, you see a couple of resistors but no large transistor and only one ferrite beads.
250407	This board originally only came in a breadbin case with dark keycaps. When you look through the cartridge port, you see a white coil housing. When you look at the tape connector, you see a large transistor but no diodes or ferrite beads.
250425	This board was delivered in both breadbin and the earliest Commodore 64C cases (PETSCII symbols on the front of the keycaps). When looking at the tape edge connector, the first component you see is a zener diode at an angle right. Behind the diode, there is the big transistor.
250466	This board only came with the early Commodore 64C with PETSCII symbols on the front of the keycaps. When looking at the tape edge connector, you see the large transistor but no zener diode. When you look through the cartridge connector, you don’s see the white coil housing
250469	This board came with the later Commodore 64C models with the PETSCII-symbols on top of the keycaps, but to confuse things, the Aldi-variant (original, brown breadbin case, but with light keycaps) and C64G (the serial number label says “C64G”) also came with this board. When looking through the tape connector, you see foure ceramic capacitors lined up with the edge connector pads.

The above method works in 99% of the cases, but you should keep in mind that Commodore is known to have been using whatever parts were around during the assembly process. On top of that, many machines have been repaired or modified at some point, meaning that you can find nearly any combination of keyboards, cases and mainboards out in the wild.