Identifying Latvian Stamp - Wm And Size

This stamp has strange WM on it's back. It looks like hollow letters.
Judging from the cancelation date, I guess it is Mi:LV 104, Sn:LV 120, Yt:LV 100, AFA:LV 99, Issued on 1925-05-11.
So, what do you make of this WM? I took a frontal pic so you'll see it's not from the cancelation.





2) I didn't find any info on different stamp size, yet I have this next pic. What do you think?

Hi.
1) according to Michel catalog,complete sheets of that stamp (and a few more from Latvia) were watermarked,in addition to the usual wavy lines, PACTIEN LIGAT MILLS 1858 (the paper factory was located in Ligatne)
On single stamps you may read a fragment of that inscription.
https://goscf.com/t/54276
2) I think those are two different stamps (Michel # 120 and 104?)

Thanks, vayolene.
1) That would really explain what I saw.
2) That is the most logical option. But, I didn't see any mention of a size difference, not in my simplified Scott and SG catalogs, not in Big blue and SCW sites, and not in stampworld and another online catalog. Since they have various perforations, can it be they had arbitrary sizes as well?

The sizes of the stamps remain the same. It's all down to the trimming of the sheets and their subsequent placement prior to printing. These things often slip through the net and have done since watermarked paper was introduced.

A marginal multiple showing a good portion of the paper manufacturer's watermark will definitely command a good premium over its catalogue value, but a single stamp will only appeal to the collector of curios. Worth keeping, though.

Thanks Tim. Still, did anyone see any reference to different sizes in these series? I failed to find any.