By personal experience of putting a file to the steel. I've done 3 vintage Swedish axes in the past couple months - two of them GB's. I found the steel to be on par with axes marked 'Collins', 'Collins Homestead', 'True Temper' and 'Craftsman' but softer than premium American axes from the same makers like 'Collins Legitimus', 'True Temper Flint Edge' or 'Kelly Perfect'.
Is that scientific? No. But once you've sharpened a hundred axes you start to get a feel for these things.
I prefer this type of data to the data I use at work as a scientist...often it is more reliable.