Linux version number convention

The correct version is “0.01”, as used in the tarball at the time (available here) and in the release notes.

Let's do some archaeology. Torvald's 1991 paper, LINUX--a free unix-386 kernel (which you can read online here) discusses version 0.03 as can be seen in the list of files in the described FTP directory (emphasis mine):

• linux-0.03.tar.Z--complete source to the operating system, in a 16-bit compressed tar archive. [. . . ]
• README, RELNOTES-0.01, INSTALLATION--ascii files containing some (somewhat out-of-date) information about LINUX.

Also note that it includes the release notes of the first version which is indeed referred to using 0.01. Also, according to Wikipedia:

By September 1991, version 0.01 of the Linux kernel was released on the FTP server (ftp.funet.fi) of the Finnish University and Research Network (FUNET). It had 10,239 lines of code. In October 1991, version 0.02 of the Linux kernel was released.[2]

This cites a message by Torvalds posted in October 1991 to the comp.os.minix newsgroup which describes version 0.02:

As I mentioned a month(?) ago, I'm working on a free version of a minix-lookalike for AT-386 computers. It has finally reached the stage where it's even usable (though may not be depending on what you want), and I am willing to put out the sources for wider distribution. It is just version 0.02 (+1 (very small) patch already), but I've successfully run bash/gcc/gnu-make/gnu-sed/compress etc under it.

Finally, the first version of the Kernel is apparently still available in its original home at http://www.nic.funet.fi/pub/Linux/kernel/Historic/. The file you can download there is:

linux-0.01.tar.gz

So, since we can still see a 0.01 file and have evidence that the second version was 0.02, it seems pretty clear that the first version was indeed 0.01 and not 0.0.1.