I have two of Celestions G18-500 speakers.It is interesting,that any place on the internet i cant find any information about speakers.The 18 is the inch marker,the 500 is the power capacity in watts I think.
I wrote an e-mail to the Celestion ,with this answer:

Sorry, we don't have any information on this speaker. As you say the =
name indicates that it is a 500W speaker. I would think it dates from
the early 1980's or late 70's as after this the name would have changed =
to a BX18.
Sorry it's not much help.
The Doc

So if anybody know anything about speakers,please write to me.

Thanks:Zoltán