I reports a lens serial number to the camera. It does provide an lens serial number to the camera. It does not report a serial number to the camera. It appears to be when that lens model was introduced. Please note that the actual date of manufacture of the lens does not seem to affect whether the lens provides an internal serial number or not. There also seems to be at least some cases where the same external serial numbers have been used for two different lenses in two different model lines. Canon probably uses a separate 'internal' serial number in order to standardize the format for all of their various lens lines, which can vary significantly in terms of the number of digits included in the external lens serial number. At the very least, it is almost certainly a number that is unique for all copies of that particular lens model. It is presumed by many to be a number that is uniquely assigned to only one lens ever made by Canon. This 'internal' serial number is a different number from the external serial number printed on the lens barrel or flange mounting ring, even when the internal number that appears to be at least partially in hex format is converted to decimal. The Canon EOS 5D Mark III reports the internal 'Lens serial number' as '0000000000'. My current copy was manufactured in September 2016. The EF 135mm f/2 L is a lens model first released in 1996. This is even the case with a newly made copy of an older lens design. When an older lens model that does not communicate an internal lens ID to the camera is used on one of the newer cameras, the field is reported as '0000000000' (ten consecutive 'zeros').

My older Canon EOS 7D and Canon EOS 5D Mark II do not include the field, even when the same lenses are used on both sets of cameras.

The 5D Mark III I use regularly also includes the internal lens serial number in the EXIF info.

The screenshot above was taken from an image captured with a Canon EOS 7D Mark II using an EF 70-200mm f/2.8 L IS II lens manufactured in 2010. Using other third party applications, such as 'Irfanview', does show the lens ID number included in the EXIF info for cameras with AFMA that can discriminate between two copies of the same lens model. Canon's own 'Digital Photo Professional' does not display the lens ID number, either. When using Adobe products (such as Lightroom or ACR), the field is never included in the EXIF info displayed by the application. Apparently Canon only started including an 'internal' lens serial number field in the EXIF info when they started including the ability for cameras with AFMA (Autofocus Micro Adjustment) to discriminate between two copies of the same lens model.

Lightroom 6 serial number