A couple of thoughts to consider Paul.
The hole in the top could be for two purposes;
one is to fill it (pretty obvious really) and the other is to control the flow:
If you put water in it and the start to pour, then place your thumb over the hole, the flow will stop. We used to use a more refined and modern item like this for irrigating eyes....called an undine.
It could also have been used as an infant feeder and you might like to ask this group of collectors
http://acif.org/They seem to have every infant feeding bottle known to man!! (or woman!)
Just as a point of interest....I used to be a designer for a baby feeding bottle company (within my nursing remit :srn:) and designed the 'Avent' Baby Feeding Bottle.....it took the company over 10 years to be brave enough to produce it, it was considerered too way out at the time!!
I had a huge collection of old bottles, but never any like this one!
It could well be that it is an invalid feeder.....