Hi, thanks for the comment! My reasoning behind this was that the lot doesn't include any letters. Some lots may be divided to e.g. 2b and 2a. In that case you are correct, we can add letters and character to the addr:housenumber tag, but from my source the lots don't seem to have any letters.

I use Turku Opaskartta/Kaavakartta and MML as a source. I speak Finnish/English, if you are more comfortable in either one.