Thanks for all your comments.
Indeed I had already checked in the excellent book from Josef Mötz (vol. 2), however for the numerous Bulgarian dummies, he gives no details on any specific pattern for rifles or for MGs.
I have also checked John Munnery’s study dedicated to Bulgarian cartridges. He also describes many variants for the dummies, and provides a checklist of observed headstamps as a function of the dummy pattern. However, in this reference also, there is no information about any specific use of each type of dummy.
This question remains then open to future investigations, pending perhaps some data from Bulgaria ?
Yes Dutch, the case of the red-wood dummy is indeed by Polte. John Munnery indicates that both ball, various dummies and blanks have been observed with Polte cases dated 1917. At least months 1,2,6,8,9,10,12 for 1917 have been observed. John also precises that some of these Polte cases used with the red-wood dummy have an empty primer pocket which would account for a dedicated production of dummies and not only the rework of fired cases.
Armament material deliveries from Germany to Bulgaria after 1916 are known and even different Maxim machine guns belts manufactured in Germany in 1916 and 1917 for export to Bulgaria are observed from specimens.
Note that my red-wood dummies with Polte cases were packed loose by 10 in a paper wrapper tied with a string.