I have noticed that some tenders have the holes on the top (whistle or not) in various places. Some are forward, others are rear. I have a feeling people are just not putting the covers on correctly. an example would be a 1689. Where should they be?
There is a 50/50 chance on how the shell fits. Most pictures on image search show the front. There can always be exceptions. There is no hard fast rule. Welcome to toy trains. 1654 on tandem shows the back.