700c and 27" can use the same tubes. In fact, many tubes say this in the sizing specs.

There are others more experienced who can confirm this, but I am pretty sure that fitting is for grease. The idea is that you stick a grease gun on that fitting and pump grease in, squishing water, dirt and old grease out of the bearings, replacing with the fresh, clean grease.

FWIW, there is not a lot of difference between 27 and 700c wheels. 27" rims are 630 mm in diameter, 700c are 622 mm. This means that you only need to be able to lower the brake pads 4 mm to be able to use a 700c wheels. Most older brakes had plenty of adjustment available.
Regards,
Raymond