This is an Olympic version of FED 4b, also known as 4L (for 61 L/D lens).
With the last version of the black cover with horizontal and vertical stripes.
It's sad that FED decided to stamp 6-digit serials on the bottom plate of the back door, because those doors can be accidently changed easily.
FED-3, Fed-4 and Fed-5 doors are the same, stamping the serial on the door might have simplified production.
There are big stamped serials (around 21mm long for 6 digits)
and small serials (around 13mm long for 6 digits).