Unlocked or not

[Sid]

I am now answering my own question ! hopefully it will help someone else in the future.

Having now taken off the combination plate holding the four dials I can understand why I could hear no clicking while turning the dials and it is because the inside part of each dial has only one groove. Thus in order for the combination bar to drop, one must enter correctly the four ( secret ) letters which on this lock happens to be ROSA. Once the bar has dropped, the safe can be opened with the key.
If the bar is then pushed down a little further, 4 circular leaf springs are raised thus retracting a pin on each and thereby the lettered dials are disengaged allowing one to change the combination letters. Simple but jolly clever and still in good order after perhaps 120 years.