Here you see a particular stamp: a postmark of Riga, 20.4.1913, and a strange perforation ‘M.P.24’.
In an old number of Rossica I found the solution:
The perforation marks of Moscow / by Gary Combs, Dick Scheper and Noel Warr. – In: Rossica 2004 ; no. 142. – p. 30-39.
Here -on the backside you see the perforation more clear. This ‘perfin’ or ‘perforated initial’ is not a company-perforation – to prevent theft by the own employees-, but a perforation of the post to prevent re-use of the stamp.
On money transfers were used the higher values, so there came a new instruction in circular 22 of 9 april 1910: the stamps had to be destroyed in the office of delivery, before sending the formulars to the Control Office.
The stamps had to be perforated or intersect. There was no obligation to make perforations with initials: every office, where the money was paid, was free to use their imagination. Here we have the perforation of a Moscow Postoffice (M.P.), a sub-office using number 24. So you find on the stamp the postmark of Riga, from where the money(formular) is send in combination with the perforation of a Moscow postoffice, where the money is paid. Details you find in the article, mentioned above, and based about this a short article about this stamp in HBG 2005 ; 47. – p. 57.