The T38 Ir has taken over from the T20 Ir.
Mainly because the front lens and pill are easier to remove than the T20.
Both the T20 and the T38 usually have the same lens and Ir LED which allows for a more user friendly option than the preset 501 that does not have the quick twist zoom function or the T20/T38.
Ideally you want one with a 3 mode driver so you can control the output so you dont over power your night vision device with Ir light.
The new, more powerful Oslon 4715AS Ir LED has been released but iv not seen them built into the any T20's or T38's on sale on ebay yet.