The closer (if installed) will have to be taken down, out of the frame, to see the numbers. 

During the week of April 17th, 2017 the 2030, 5030, and 6030 cylinders are stamped with a six digit number.  First number is closer size, second is shift, and the last four are date of manufacture-week and year. For example see here.                                       

On the older models, shipped prior to week of April 17, 2017 the date stamp and size could be in different locations depending on the age of the product.  These stamps will be on a smooth machined surface of the cylinder.  Any Stamps on a Non Machined surface of the cylinder, are just casting numbers.

• For example - in below images the date or size stamp is on machined surface (in RED), and the blue circled numbers are on a non-machined surface and are just cylinder casting numbers.
• Or if unit is very old the date stamp can be on clip that holds the spring tube of the closer to the plate.
• The size may also be located on the corner of the closer body.  See here.