What fits in a mint tin and lets the 19th Century communicate with the 21st?
It's not a real riddle, it's the Morse key USB keyboard! A nifty little interface that connects to a telegraph key on one end and on the other, tells your computer it's a USB keyboard.
While first at glance you might see only hobbyist fun (and big fun indeed for some of us), there's a serious side to this kind of interface, too: persons with limited mobility or strength who are unable to use a conventional keyboard can use a telegraph key. It has been done before, usually in the form of software running on a dedicated machine with a specialized interface; this is plug'n'play, portable. USB connection, Dr. Hawking?
Downside, you'll need the hardware/software to flash an Arduino-compatible microcontroller. Or know someone who can.
Update: If you'd like to match this to a landline telegraph sounder, Jake von Slatt's sounders are a scratch-built solution; the morse2led software he uses to drive them (from a keyboard led) would also work a commercial sounder, piezo buzzer or electromagnetic buzzer, though you might have to fiddle with the drive ciruits a little.
If you were wondering, yes, there is Morse telegraphy on the Internet. The software is free. The linked page, at the wonderful Morse Telegraph Club website, will guide you to sites and software, some using landline (or American) Morse and others using radio (or International) telegraph codes