From the book, "Crown Jewels: the Mauser of Sweden" - by Dana Jones, "HK" was inspector Helge Kolthoff at the Carl Gustaf state armoury from April 1, 1912 to Feb. 28, 1923. During that time, that Armoury made serial numbers 468### m96 rifles during the year 1919 - so that receiver started life as a Model 1896 long rifle, when it was made in 1919 - or at least a rifle with that serial number did. If that was an original, it would have had a crest on the receiver, etc. which you say is now all gone. I have a much later made Husqvarna sporting rifle that has no crest either - but has no serial number, and so on - so is not the same origin as yours. It will be for someone else to tell you the significance of the words "Gevarm Aktori" - I have not come across them. I do not know about no "Nitro" word, but I do not think any military Swede barrel had the caliber or cartridge marked on it when it was made - certainly none of the five or six here are marked, other than a Swede Crown symbol on the very bottom where the barrel screws into the receiver.
I do not think many countries marked the cartridge or chambering when the rifle was in service - most Lee Enfield No. 1, No. 4 and No. 5, Enfield P14 and P17 that are marked are from a proof mark that was done when the arm was sold out of service - not while it was in service.
As far as the "safety conversion" - the original was a three position lever that swung over the top - was several companies that made two position up-down scope friendly replacements - most of them gave up the middle position function of the original - so Timney, Buehler, and Dayton Triaster are some brands that I have bought over past 25 years or so - sold by Brownells, among other places. Usually they were up/down levers along right side of the scope eyepiece, but I know Parker Hale and FN made similar that went up/down on the left side of the scope for Mauser 98, so I would guess someone made similar for m96 as well. So about anyone could buy and install them if that is what you mean. The original m96 bolt will have a straight handle - sticks straight out right side when bolt is closed, and straight up when bolt is open - that was usually cut off and re-welded at an angle, or forged, to change that so that it would work with a scope - as it was, it worked very nicely - safety plus straight bolt handle plus barrel mounted iron sights - they do not work very well, at all, with a scope. The original straight handle bolt will have a "straight" Swede crown on bottom of the ball, and top side of that ball will have the last three digits of the receiver's serial number stamped on it. So will the safety lever, bolt shroud and rear end of the cocking piece - if those are original matching military parts.
I believe the Swede state armoury called Carl Gustaf made circa 500,000 m96 rifles - plus the ones made in Germany for Sweden, plus the m94 that were made, plus the m38 and m96 that were made by Husqvarna during WWII - many or most of them got sold off as surplus in the 1950's and 1960's - so likely Canada and USA were flooded with very inexpensive Swede Mausers and most everyone got to try their hand at making a decent hunting rifle from a Swede military rifle - for cheap - some were very well done and look and worked excellent - some not so much. The shortest barrelled one here measures 18 3/8" (468 mm) from closed bolt face to muzzle - from various markings, I am sure it was made from a 1943 Husqvarna m38, but except for those marking, would be nearly identical to an m96 from Carl Gustaf Armoury. I think the longest sporter barrelled one is close to 26" long barrel and the originals, I think, are 29 inches long barrel - all were 6.5x55 chambering though.