Gerry,
The engraving patterns suggests a model 550, or a model 578.
But ultimately the only way to know for sure what model it is would be to ask Arrieta. Arrieta, like just about all the makers, will put any engraving pattern the customer wants on any gun he is willing to pay for.
As far as using the number of visible pins in the locks to determine model, I can only remark that there are as many Arrieta model 578 guns out there with seven pin locks as with five pin locks :-)
But buy the gun rather than the name in any case. That makes model moot.
