The part number ending with the equal sign means it's spare. The other one can be ordered as part of a configuration only.
GLC-LH-SM can be replaced with GLC-LH-SMD=.
If you need replacement parts you'll have to go through Cisco (or your Cisco partner) to get an answer on whether it can be replaced. We are not Cisco employees with the authority to approve replacements or even consult on whether your hardware qualifies for replacement.
**Please remember to rate helpful posts as well as mark the question as 'answered' once your issue is resolved. This will help others to find your solution faster.
The part number ending with "=" is a spare, yes.
If your running product fails and you don't have Smartnet support contract, you would have to order a new one and you would use that part number for the spare part.
There is no physical product difference, it is just a stocking part number distinction within Cisco.