Go in and talk with the manager, or if possible and better, the owner. Be calm, don't be accusing despite the situation, be reasonable. Explain what happened. Act like someone just made a mistake. If the manager tries to dismiss you, get firmer. That's when you say you will file a claim of theft with the police if they really want to force you to go that route, but you'd much rather the manager just check around and find your original Artley silver head. If manager still refuses, then call the police on the spot. And if there's a news station in your city, esp one that has a Call for Action/Consumer Help feature, call them after the police. Tell them your situation. Make sure the manager hears you doing all this. If manager still does not try to produce your original head, then sit there remaining polite until the police and hopefully the news camera/reporter all show.
Do NOT get hysterical, loud, angry, hostile, or threatening (esp never the last). All any of that does is makes the store person, guilty or not, less willing to help or to "happen to find" it. Always remain courteous, polite, calm, reasonable.
Please keep us posted how this goes for you. Fingers crossed you get your original head returned.
1975 Gemeinhardt M2 in chrome nickel
1982 (app) Armstrong 80
2006 Yamaha 584
1978 Artley piece of crap 15 P
1982 Gemeinhardt 4S
1980s (?) Armstrong all wood (no model)
2006 Jupiter di Medici G0199
Plus many many flute-cousins....