Charles Letts & Co. Ltd. Gold electroplated. 1.1mm lead propel-repell
Engraved:
Charles Letts & Co. Ltd. Gold electroplated
London and Edinburgh, Made in England
Common Vintage Lead Sizes
The most common sizes of lead for vintage mechanical pencils were 1.1mm and 0.9mm. 1.1mm leads were frequently referred to as "thick" lead and the 0.9mm size was often referred to as "thin" lead. Of course, today, there are much thinner diameter leads made, but we're talking antique pencil leads!
For your reference:
1.1mm = 0.046 inches
0.9mm = 0.036 inches