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Why is a "T" or an "O" marked on each side of the SWISS MADE label at the 6 o'clock position on some watches? (example: T Swiss Made T)............well, here the answer:
http://www.ozdoba.net/swisswatch/swiss_to.html
Here is the "T" ...
On some watches, you will find a "T" on the dial, usually to both sides of the "Swiss Made" notice at the 6 o'clock position. Typically, it looks like this:
Fortis Official Cosmonauts Chronograph GMT
This "T" indicates that radioactive material, i.e., tritium (that's why it is a "T") is used in this watch. Don't worry: the radiation will definitely not be harmful to the person wearing this watch. The "activity" used in watchmaking is limited to a value of 25 mCi (milliCurie) which means that no radiation will pass through glass and/or case of this watch.
Some manufacturers make a special point of mentioning that they do not use the maximum allowed activity and put additional information on the dial:
"T < 25" means that the activity of the luminescent material is below the legal limit.
If, on the other hand, you find "T 25" on a watch, the manufacturer informs you that he has used the highest allowed activity; this is sometimes necessary for special purpose watches, esp. for deep-sea diving.
If you need a dial or hand replacement for such a "T" watch, it will be necessary to send it to the factory; handling and working with tritium requires special safety precautions and permissions that are usually beyond a "normal" watchmaker's possibilities. Tritium-filled hands are likewise usually not available from watch parts specialists ("fournituriers").
... and here is the "O"
Patek Philippe Ellipse
I asked about this symbol at the Basel '98 show: At Patek Philippe, nobody could tell me what it means - the sales staff present didn't know, and there was none of the watchmaking professionals at the show (I guess these specialists have better things to do than to attend shows ...).
At the Perrelet booth, I was lucky to find a watchmaker who could solve the riddle: It is likewise an "official," though nowadays seldom used symbol which indicates that the indices on the dial are made of gold. The letter "o" is used as it is the first letter of the word "or," the French word for gold.
According to information from Zenith, it is not an "o," but the Greek letter "sigma;" in this case, the definition with "or" would, of course, not be applicable. Zenith confirmed that it indicates golden indices on the dial so that we can consider this settled.
Copyright © 1997-2002 and responsible for contents: Christoph Ozdoba.
First version 1997; last edited: March 28, 2002.