Many of us take it as a given that the ion arm rod on the Spengler pack is bent based on all the reference pictures in the library, even the photos taken after shooting Ghostbusters show the original 84 Spengler pack with a bent ion rod.
I had pretty much decided to bend my rod, but then decided to pay closer attention to what is actually shown on screen for 84. The first good shots available of ion rod are in the Sedgwick Hotel scenes, to my somewhat surprise it is straight!
The next opportunity takes place when the Ghostbusters are climbing the stairs in Dana’s building and then they question where the stairs lead - up. There it is seen that the ion rod is bent. However the next appearance in the rooftop scene it is straight again!
We also get a good shot right before the guys are about to cross the streams on the rooftop, it is still straight.
It is seen again after the defeat of Stay Puft, it seems that Ramis may be wearing the hero pack as the straight rod can be clearly made out - other scenes that show the guys wearing the stunt packs show they are clearly missing the rods. Even if this is a stunt pack with a rod added, it is clearly straight.
The next good shot we get is in the end where Janine runs up to hug Egon. Not readily seen, but with some adjustment to the photo settings we can again clearly make out the straight rod.
EDIT : It was brought to my attention that the pack Egon is wearing when hugging Janine is the Venkman. I can make out the top of the ion arm resistor as white, which is not true to the Spengler and the back of the rear cylinder on the thrower is flat. There are a few other differences too, but it seems the idea of the straight rod still carries through.
All this said, the shots were obviously filmed out of sequence with the stair scene being filmed last - something happened to the prop before that was filmed to bend the rod. So, which is “correct”? It would seem both, but the straight rod gets far more screen time than the bent one. With that, I have decided (and justified to myself) to keep the rod straight on my build as it is specifically 84.