How these stamps were used, or rather not used, and finally used up, is a fascinating question.
Firstly why were they introduced at all? Yugoslavia had done perfectly well without Service stamps for 20 years and never issued them again. I suspect that the answer lies in WW2 when both German occupied Serbia and 'independent' (but effectively German occupied) Croatia made use of Official stamps, which had, of course, been widely used in Germany. So by 1946 the bureaucracy and postal service of half Yugoslavia had become used to this kind of issue.
However very little use seems to have been made of them in government offices in 1947 (the only full year in which they were in official service use) and correctly used covers emanating from government departments accompanied by a SLUZBENO ('Official') handstamp are scarce.


By 1948 they has ceased to be used for their original purpose and were made available for Post Offices to use for 'in house' purposes. Hence they were used for
Up-rating postal stationery to a new tariff before sale to the public

Registration

Postage to abroad

There are very rare examples of them apparently being used as ordinary internal postage; it is impossible to be certain whether these were stuck on by a postmaster in the post office or whether they were sold over the counter, perhaps as the result of shortages of certain values. I suspect the former.
On 8th Feb. 1947 remaining stocks of the ˝ dinar stamp were surcharged in 5 various values for use as emergency ordinary postage stamps in the disputed territories on the Istrian coast under administration by the Yugoslav Army on behalf of the Allies for the past year and a half, pending a decision on whether it was to become part of Italy or Yugoslavia. (A decision which was to be made very soon – hence no point in creating another new set of stamps. But something was needed temporarily because there was a postal rate rise on this day and 3, 6, 15, and 50 dinar values were not available from the previous issue.)


In early Oct. 1949 remaining stocks of the 8 dinar and 12 dinar stamps were surcharged for use as ordinary 3 dinar postage stamps. Ironically this cover was sent by a Court, which would probably have previously had the right to use Service issues.

Subsequently Government covers carrying the SLUZBENO mark could be sent without a stamp affixed, and no more Official stamps were ever issued.

So all those of you who have the full set of these, keep your eyes open for one used on a Sluzbeno cover in 1947 to display with them, and impress your friends!