'but also's'

Hi,
During the past seventy-five years since Japan's closed doors were opened, the Japanese have been described in the most fantastic series of 'but also's' ever used for any nation of the world.

Here's an excerpt from The Chrysanthemum and the Sword. I don't quite understand what the three quotation marks mean in 'but also's' . Does it stand for 'but also is'？My understanding is that “s” here means the plural form of 'but also', but if so, what does the last quotation mark mean？

The two outer quotes are there because but and also make up a noun. However, it isn't a normal noun. It means you can make up sentences of the form The Japanese are not only X but also Y.
The inner quote is there because also is not normally a noun and so the plural is artificial or unusual. The author could have made the outer quotes double quotes to distinguish the from the inner quote more easily.
(You often see a quote in an unusual plural. There's the phrase the "do's and don't's" - things you should and shouldn't do. The "do's and don't's" of handling dangerous chemicals. There's the phrase "what if". My life has been a series of "what if's" - what if I had done this, what if I had done that.)

rhitagawr said:

The two outer quotes are there because but and also make up a noun. However, it isn't a normal noun. It means you can make up sentences of the form The Japanese are not only X but also Y.
The inner quote is there because also is not a noun and the plural is artificial or unusual. The author could have made the outer quotes double quotes to distinguish the from the inner quote more easily.
(You often see a quote in an unusual plural. There's the phrase the "do's and don't's" - thing you should and shouldn't do. The "do's and don't's" of handling dangerous chemicals. There's the phrase "what if". My life has been a series of "what if's" - what if I had done this, what if I had done that.)

Thank you so much for your detailed explanation. I've learned a lot from your answer.

Glad to help.