It's there on every stamp of the unoverprinted 1886-91 issue (this is Scott 45) for security purposes as a check against forgeries, likely more for making sure that forgeries were not swapped out in stocks of sheets. Typography was a common printing method not just for stamps and the earlier issues could have been forged. By this time, most countries used watermarks as a security check but Sweden for some reason had never used watermarks. It is rather cheesy since it could be easily forged itself but it was an extra step and meant more cost for any possible forgers. So this blue crown backprint might have just been a stopgap measure.
Note that the following issue was engraved for any value of consequence (5 ore and up) – much harder and expensive to forge. With the following 1910-14 series, Swedish stamps were again typographed but now finally had watermarks.
The Post-Horns were printed on the back for accounting reasons;and paper consumtion by the'Bagge'printers.The larger posthorns was also in the the corners of the sheet.These post-horns were forerunners of the watermark that was introduced in the next 1891 Oscar II issue in a copper matrice.