I overpaid for this... IBM Model M ashtray, for the lack of a better word. I looked at pictures of the internal assembly online, and I think there're missing springs? For example, I can't press the upper half of the enter key because there's just no spring there, not even blue placeholders. I can't exactly buy new springs from Unicomp as the shipping is so prohibitively expensive that I could buy a IBM Model M from someone else.
Each keycap has one spring. The larger keys have a stabilizer (the blue insert). From your picture, it looks like you have all the springs. You'll just need a blue insert for enter key, + key, and numpad enter key.
You may be able to find someone selling the stabilizer inserts on ebay for cheap. In the meantime, you can move the blue insert from numpad zero to the enter key if you use enter key more often.
Good to know! What I also didn't know is that the spring and stabilizer placement depends on the keyboard layout. My enter, numpad plus, and numpad enter key have what I now assume are metal stabilizer bars. So I guess everything was alright after all, or do these still need stabilizer inserts? I think it's weird that I can't press the upper half of the enter key, though.
I'm not all to familiar with the iso layouts. Non of my model Ms have wire stabilizer for any key other than spacebar.
Maybe your enter key is using a wire stabilizer and the wire is missing. Check the bottom of your enter key and see which type of stabilization it is using (https://geekhack.org/index.php?action=dlattach;topic=55003.0;attach=56349;image). If it is using the wire and it's missing or damaged, you can probably take the wire from the plus key and use it for enter
Some early Ms have extra wire stabilizers on some longer keys. I think I've seen it on the numpad plus and enter on like '86 and '87 models, but it disappeared on later models. I think this might have corresponded with using more plug inserts.
It's the ISO enter key with stabilizer judging by that image.
But I found an interesting reference that suggests that everything here works as intended.
Image 1: 122-key IBM Model M
This layout matches mine with the following exceptions:
Image 2: 122-key IBM Model M internal assembly
If you look at the placement of stabilizer inserts, then this matches my IBM Model M. Same exceptions apply. I couldn't determine whether the 122-key IBM Model M enter key also uses a stabilizer metal bar, but it's suspiciously similar to my layout nonetheless.