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 :-)
Please show us a few of those Arrieta 578 guns with seven pin locks, many of us would like to see them. Pics or simple links to them would be fine. Oh, by the way, make sure that those that you come up with aren't 803s or others, misidentified as 578s like this one:
http://www.gunsinternational.com/Arrieta-578-Sidelock-Ejector-16-Bore.cfm?gun_id=100514688
