It's a common way to carry chem lights to ID friendlies at night. The empty bottles provide a simple, clear, waterproof container that can be taped or attached to the aerial - The chem lights can be replaced quickly and easily just by unscrewing the cap of the bottle.
In 1/35 there are a couple of simple ways to replicate the water bottle/chem light.
For a previous project I used a cut-down Academy water bottle from their generic modern clear sprue that they supply in some kits (cut down from the '2 litre' to represent a '1 litre' bottle).
I drilled a hole down into the bottle from the cap end and then filled it with paint (carefully, to avoid getting it on the outside of the bottle). Finally I painted the 'cap' in blue. The whole thing sits upside down on the aerial and was attached with a couple of thin strips of Tamiya tape (pre-painted and cut to length)