For hard drives it was complicated, since their size is a function of the number of cylinders, heads, and sectors. Since the sector size is normally 512 bytes, they were a multiple of that. See e.g. this table of old hard drives:

https://www.computerhope.com/hdquantu.htm

E.g., if you take the Quantum Bigfoot 1.2 GB model:

2492 cylinders * 16 heads * 63 sectors * 512 bytes per sector = 1286111232 bytes

The size it was sold as, is definitely closer GiB (as in power of two):

1.2 * 2*30 = 1288490189

With rounding, they could sell it as 1.3 GiB with the current marketing trick of using SI units.

So the old branding strategy seems to have been: compute the size in bytes, then find a reasonably MiB or GiB value (powers of two).

The whole power-of-10 thing for storage goes back forever-- to IBM mainframes and to the first hard drives used in PCs.

E.g. Seagate ST-506, first 5.25" hard drive.

"The total formatted capacity of the four heads and surfaces is 5/10 megabytes (32 sectors per track, 256 bytes per sector, 612/1224 tracks)."

612 * 32 * 256 = 5013504, or 5.01 megabytes or 4.78 mebibytes.

The gap between SI prefixes and their base-2 approximations grows as sizes get larger.

Before 1GB HDD’s the file system overhead was a much more dramatic effect so it didn’t really matter to consumers which prefix convention they used or how they rounded.

As I see it the GB numbers are powers of 1000 rounded down:

2492 * 16 * 63 * 512 / 1000 ^ 3 = 1.286111232

4092 * 16 * 63 * 512 / 1000 ^ 3 = 2.111864832

4994 * 16 * 63 * 512 / 1000 ^ 3 = 2.577383424

Using GiB you get different numbers if you look past the first element in the table:

2492 * 16 * 63 * 512 / 1024 ^ 3 = 1.197784423828125

4092 * 16 * 63 * 512 / 1024 ^ 3 = 1.966827392578125

4994 * 16* 63 * 512 / 1024 ^ 3 = 2.4003753662109375

So I stand by my point, they have always used SI units. It's only ram (and software) that used incorrect definitions of k, M, G and T.