License ambiguity - When license version is not specified by author?

Question

I am tasked with making a due diligence of legacy software. Part of this is to reassess all software dependencies and their licensing. However, there are various ambiguities, occurring most commonly in old code.

I am now facing this particular issue: I have a dependency that I have seen to be indicated in the code to be licensed under L-GPL. With a fair amount of certainty this can be identified as the GNU Lesser GPL. However, I cannot find information about the version this refers to.

My question is what version should I assume, and I can think of several options:

1. The version of the time of creation of the software, namely LGPL 2.1, or
2. The version of the time of our use of the software, which practically would allow me to choose today's most recent version, i.e. LGPL 3.0.

To my understanding the difference between the two is negligible, but the combination of legal and technical context urges me to try to be specific as much as possible.

Please, excuse me if this question has already been asked in some other form and I haven't been able to identify it with my searches.

Answer 1

Since version 3 didn't exist when the software was written, the original author presumably intended that 2.1 was used.

However, point 13 of LGPL 2.1 states (emphasis mine):

The Free Software Foundation may publish revised and/or new versions of the Lesser General Public License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns.

Each version is given a distinguishing version number. If the Library specifies a version number of this License which applies to it and "any later version", you have the option of following the terms and conditions either of that version or of any later version published by the Free Software Foundation. If the Library does not specify a license version number, you may choose any version ever published by the Free Software Foundation.

Point 6 of LGPL 3 includes a similar statement.

On this basis, and assuming the absence of any other statement in the software as to which version was intended, you should be able to use either version.