Maybe it's the open round letters which made me think it's just a bit child-like?

?
It just doesn't look like an "official pawmark".
(it evoked the same feeling in me as reading a child-like address, finished with; Europe, Planet Earth, The Solar System, The Universe....")
I may well be completely wrong!
It is, however, a very sophisticed piece for '74 - perhaps it's the mismatch between the sophistication of the piece and the unsophisticated appearance of the signature which is messing with my thinking - which is all just gut stuff.