Key code question

Hey, I have a 1973 240z and I have what I believe is the original key. I wanted to get a replacement made and the code on the key is just a 4 digit number (5XXX) with no prefix. I thought the keys generally had a prefix for the type of cut? The key does work on everything (ignition, both doors, glove box, and trunk)

I took a look and there is no glove box code sticker. I attached a picture of the key (with a repro blank I was hoping to use) to show the style (and edited it a bit to remove the last 3 code digits and some of the key length)

Any idea what might be up? The key is super worn so I was hoping to avoid having a duplicate made directly from the key