Conway Stewart used 2 different formats for the numbers engraved on the barrel.
If it was a Limited Edition pen, then the number was the Edition Number ie 10/300 means this was the 10th pen made in a limited edition of 300 pens.
However, much more commonly used was the format in the below photo which shows a strange number 058/001. The first number is the colour reference of the pen and the second number was the serial number. In this example 058 = Classic Black and 001 = this is the first pen made in that colour in that range.
The full list of colour numbers and their correlating pens can be found here.