Better to be precise about your start date, 'about 20 months' is vague and might give the recipient the impression that your not too sure! So better to say I have been employed here as a AC draughtsman since (date). Then go on to elaborate your achievements, such as In that time I have always met targets and deadlines, been punctual and had a good attendance record (hopefully!) Then outline your commitment and aspirations for the future for yourself and the company. If you have had regular meetings with your supervisor then you should mention that and that there have been no adverse issues with your work (again hopefully!)
Then say you feel it would be helpful if you could meet with the recipient to discuss your role and explore areas of opportunity that may be available. When you get to the meeting be positive and receptive. Thats when you ask for the rise.
If I got your letter I would probably ignore it. Hope this helps
Student or Learner