Well, it could be either. The 512k was released in Sep. of 1984, and the 128k was sold until October of 1985, and your machine was manufactured in May/June. Decoding of these old serial numbers is something I like to do. Here is some info on yours:
F: Fremont, Calif. (manufacture plant)
5: 1985 (year on manufacture)
21: 21st manufacturing week of the year (May/June)
M0001WP: Europen model number (same for the 512k and 128k)
Yours is going to be a tuff cookie to crack. I'm going to continue looking further into this.