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 If you're actually interested, and not just enjoying being argumentative, get off your butt and do you own research. Remember:
Google Images Is Our Friend.
Use the query string 'Arrieta 578 gun' and you will see any number of Arrieta 578 guns with seven pin locks. You can also see a page from an Arrieta catalog, showing a 578 with (gasp!) seven pin locks.
If you want to get a maker's attention about locks, forget H&H pattern locks and ask to have Purdey pattern locks on your gun. That will get them excited, and run up the cost of the gun by about 2,000 Euro.
Seriously. No one, other than the guys who don't like exposed pins in locks because it breaks up the engraving surface, care how may pins show on the outside of a H&H type of lock. Three pin, four pin, five pin, six pin, seven pin - it doesn't matter. They all work just as well.
