Well, it depends on what the label contains. In some controls, this will be Label1 and it works, in others it will not. Do not take it.
So, on a simple page, it appears as Label1:
<form id="form1" runat="server">
<div>
<asp:Label ID="Label1" Text="test" runat="server"></asp:Label>
</div>
</form>
- (ctl00_head_Label1 ), -:
<asp:Content ID="Content1" ContentPlaceHolderID="head" Runat="Server">
<asp:Label ID="Label1" Text="test" runat="server"></asp:Label>
</asp:Content>