Adam — 18 was probably the stopper number. I've seen stopper numbers in various places, such as on the top of the rim, on the inside curve of the rim, various positions on the neck, and, as with yours, on the base. Here's a set with engraved base stopper numbers:Mystery Arts & Crafts Condiment Set in Glass and EPNS (http://www.glassmessages.com/index.php/topic,22938.0.html)Engraving the stopper number on the base seems to be rather odd, as you would tend to turn the item upside down to see it, emptying the contents all over the place. However it was there primarily for stock control and despatch at the glassworks, and for the wholesaler and retailer. In this case I don't believe much thought was given to the customer who had bought more than one.
Bernard C. 8)
Paul — You might be surprised by how many members of the public notice things like stopper numbers. I can't think of any reason to distinguish glass collectors and dealers from the public in this respect, save for the fact that collectors and dealers are likely to examine an object rather more closely.
Chris — I didn't say it was a stopper number, just that it probably was. It could also have been a glassmaker's number for allocating piecework remuneration after annealing, stoppering, and decorating; or an inspector number showing that it had been passed for sale.
Had it contained a smooth island malt, I would have regarded it as a good excuse to finish off the contents before checking the base. ;D
Further observation:Notice that the "1" in "18" is not a Continental "1".Bernard C. 8)
ps — hope you like my punctuation! ;D