I'm not as familiar with the 1908 as I am the 1903....because I own a 1903. All 1903's were originally chambered for the 6.5 X 54 M/S cartridge. That some have been re-chambered and re-barreled is beyond dispute. Anyway, my "Model of 1903" was proofed in 1929 and assuredly manufactured long after 1903. The date has nothing to do with the model number. With firearms, generally, if a year model is used to identify a firearm that is the year of introduction. The Model of 1908 was introduced in 1908 and all were originally chambered for the 8 X 56 M/S cartridge. The 1905 was chambered for the 9 X 56 M/S cartridge. I expect you see how even though they are the same rifle in this instance the model number depicts the cartridge for which it was chambered in addition to the year of introduction. Clear as mud?
How long a firearm may or may not be made depends on the popularity, thus profitability, of the model.
You've surely heard of the "30-06" cartridge. That denotes 30 caliber and in this case the year that cartridge was introduced, 1906. There is absolutely no rhyme nor reason to the naming of either cartridges or firearms. A minor exception can be made for the European method such as 8 X 56 where the first number is the caliber in millimeters and the second number is the empty case length in millimeters. HOWEVER!!! By that method duplication and sometimes confusion is assured. Cartridge nomenclature is definitely as clear as mud....to everyone.
I can find no end date for the manufacture of the 1908 M/S. I expect it would be safe to assume that during WWI manufacture slowed considerably if it didn't stop completely. After the war manufacture resumed and I would expect continued up to the beginning of WWII. Some German sporting rifles were made during that war but I do not know if the same can be said of Austria. Those German sporting rifles were mostly made in small shops and as Steyr was a fairly large manufacturing concern I suspect they didn't put together many sporting rifles as their output would have more directed toward the war effort.
All the early Mannlicher/Schoenauer's were the same rifle, the 1903, '05, '08 and '10. The only difference was in the cartridge they were chambered for and some minor details in the stock and/or sighting equipment or, as I recently learned, in the take down models.
Those early models are valued as collector firearms and shooting/hunting rifles. The less they are altered from original the more valuable. Too often a valuable, original firearm has been altered by a well meaning individual who later complains it isn't worth as much as one that has not been altered or refinished or some other change. The thinking often being, "well, mine looks better" and from a purely aesthetic standpoint it might. But it is no longer original. Often 50% of the value is "refinished" off. As with vintage vehicles, they're only original once.
Get me pictures, clear pictures of all the proofs on the action and barrel and I can get pretty close to the meaning of most, if not all. Better yet, post them on the GGCA site and I know you'll receive definitive answers.
Regarding the crown. Who knows? If the rifle was indeed proofed in 1921, which as you allude is speculation on my part, I doubt that in 2 years from the abolishing of noble and/or royal rank they were quite ready to give up the title, whether there was any authority with it or not. I can easily see a noble putting the crown on a personal possession for the least of a reason that they want to be reminded of who or what they were....and that others who saw the rifle would know it also. That holds true today some 96 years after the titles were supposedly abolished. Many former nobles don't put much stock in the title but will point out that they come from royal or noble blood and "that is our coat of arms" or "our crest". Pride is a powerful emotion.
Didja hike over to the GGCA site? Those guys are all gentlemen, much the same as this site, and I know they'd help however they could.