What are "rubber notations" in "The Just Men of Cordova"?

Question

In chapter 2 of The Just Men of Cordova by Edgar Wallace, the author was describing a business man in London:

His was a name to conjure with in certain circles. In others it was never mentioned. The financial lords of the City—the Farings, the Wertheiners, the Scott-Teasons—had no official knowledge of his existence. They went about their business calmly, loaning their millions at a ridiculously small percentage, issuing Government loans, discounting bills, buying bullion, and such-like operations which filled the hours between eleven o’clock, when their electric broughams set them down in Threadneedle Street, and four o’clock, when their electric broughams picked them up again. They read of Colonel Black in their grave way, because there were days when he dominated the financial columns. They read of his mighty stock deals, of his Argentine electric deal, his rubber notations and his Canadian copper mines.

Does "in their grave way" mean "with their serious attitude"?

And what's exactly meant by "rubber notations", is it "rubber figures"?

Answer 1

The question is right that “grave” is used with the sense “dignified; serious”.

“Notations” is a typographical error for “flotations”. You can check this against page 32 of the 1931 Ward, Lock, and Co. edition on the Internet Archive:

“Rubber” is natural rubber (latex), which is used in the manufacture of pneumatic tyres and many other objects. In the early 20th century there was a rubber boom (a large expansion of rubber production) in South America, financed in part by the non-fictional equivalents of Colonel Black.

A “flotation” is “the act of getting (a company, scheme, etc.) afloat or fully started” (Oxford English Dictionary), usually referring to the raising of funds by selling shares in a stock market.

When using a public domain source of digitized texts, like Wikisource or Project Gutenberg, you always have to be aware of the possibility of typographical errors. Typically these texts are produced by starting with a scan of a printed copy of the work, using optical character recognition (OCR) software to digitize the text, and then using volunteer proof-readers to correct the mistakes made by the OCR software. It is easy for the volunteers to miss mistakes, especially when the result of the mistake is a real word, as it is in this case. See this question for another case where an OCR error yielded a real word.

You should consider correcting the error at Wikisource using the “Edit” link near the top right of the Wikisouce page.