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If it’s old and heavy it’s almost certainly a lead alloy (pewter is just one such alloy). I don’t think I’d want that in my mouth - I already have enough brain damage! 
Not lead, leastwise, mine isn’t. We’ve seen others of the type. The “RE” is believed to be the key signature. “3” is probably a maker’s model number.
Look closely at the picture. Note the ring just below the mouthpiece. If it’s like others I’ve seen, that is a solder line where a cast mouthpiece was soldered to a rolled conical tin tube. The tin is quite thick and heavy making the whistle very dent-resistant. I’ve seen claims they were military, but no proof. The mouthpiece could be pewter, but mne is plated with something lighter. Very nice. solid, even loud tone. A very good outdoor whistle, IMO.
Personally, I think we spend far too much time worrying about a lead-fippled whistle. Sure its touching your lips, but is ain’t as if you’re CHEWING on it! No one lives forever, but if you spend all your time worrying about trivialities, it’ll seem like you do.
I once had a very similar French whistle from around the same time. It was grey-ish metal (pewter/lead like) plated or at least covered by a silvery/chrome plating or paint (it was peeling a bit). Even though designated ‘RE’ it was around C sharp. Quite a nice whistle actually. Micho Russell made off with it eventually after twisting my arm relentlessly for some time.
I have read that shiny, metallic lead isn’t as dangerous, but white, powdery lead oxide that forms on the surface is extremely toxic.
I would still be cautious about playing a suspected lead fippled whistle. At the very least, I would make sure there’s no lead oxide powder anywhere and all the metal surfaces are clean, metallic looking. Further, if I were expecting to play such a whistle very much, I would coat the suspect parts with a thin lacquer. And I would be especially careful not to pull any condensation forming in the windway back into my mouth.
Best wishes,
Jerry