I think you mean Shaw, not Sharp. The oldest ones had a plug made of lead.

The best whistle I've ever had was an "Atlas", made in Paris, probably before WW2. It was a very thin moulding of cream phenolic resin, streaky like piano keys, reverse conical bore, with the holes surrounded by raised bumps. I lost it when I was robbed in Dublin. The thug probably threw it in the bin.