Dear Jimmy,
The question that comes to mind is whether this employee took proper clearance before leaving the company. If yes, then why was the recovery process not initiated at that time? Or is it possible that the employee abandoned their employment? If the latter is the case, then you can only send a letter to the employee's home address. If it is the former, then disciplinary action should be taken against the employee who allowed this oversight.
While I am not suggesting that you should not send the letter to this person, they should have returned the company items regardless of being asked. However, the content of the letter would depend on two factors: (a) the employee's abandonment of employment or (b) if the separation process was normal but the recovery of company items was overlooked during clearance.
Thanks,
Dinesh V Divekar