DEC pack, unpack and disk-images

[Gribouillis]

does that denote an initialize string of bytes or something else?

Yes the b in front of a literal string denotes a string of bytes

"ABC" # this is a string of unicode characters (type str)
'ABC'  # this is also a string of unicode characters (type str)
b'ABC' # this is a string of bytes (type bytes)

A string of bytes behaves like a read-only array of 8 bits integers

>>> list( b'tom' )
[116, 111, 109]
>>> "tom".encode('ascii')
b'tom'
>>> b'tom'.decode('ascii')
'tom'
>>> 

because I don’t learn anything, which was the point of my post.

Your main problem will be the time it takes to get familiar with Python's libraries. In this case I want to use the bytes.translate() method because then I don't have to iterate through the bytes of the string (more precisely the iteration is done by the underlying C implementation of Python), so I first create a 256 bytes translation table that I need to use this function.

Here is a version of unpack()

rev_table = r50 + b'\xff' * (256 - 40)

def unpack(n):
    if 0 <= n < 64000:
        a, c = divmod(n, 40)
        a, b = divmod(a, 40)
        return bytes((a, b, c)).translate(rev_table)
    raise ValueError(n)

In Python it is better to raise exceptions than to return error codes (because error codes need more ifs for the caller while exceptions propagate by themselves)