Pistols were repaired and had parts replaced in piecemeal fashion, without any rebuild marks.
I have a M1911 that my dad bought from Uncle Sam in the '60s, shipped straight from a government arsenal, with the original blued finish but with a WWII-era replacement barrel and M1911A1 grip safety.
Your slide might be original to the gun. There were two features changed during 1918, the location of the rampant Colt and the shape of the "stirrup cuts" on the slide.
Most sources say that somewhere around serial number 275k, the horse was moved from the rear of the slide to the middle, and the stirrup cuts changed to the longer, sloping profile still used on Colts today. For a period of time, slides were made with a mix of features, so your 290k serial *might* correctly have a slide with late, early, or mixed features. My 296k has the horse in the middle, but the early stirrup cuts, but I think my serial is probably too high for the slide to be original to the frame.
Your gun looks refinished, and a fairly certain way to tell is to look at the polishing "grain" on the frame. Originally, the frame was polished fore-aft above the mag release, and polished up-down below, so there is a very visible change in the pattern; if the polish is fore-aft overall, it's a reblue.
EDIT: After looking at more pics . . . the slide shows early features front and rear, but Colt was making 1000 pistols a day at that time, so a feature change that came within a certain serial range could mean the difference between Monday and Friday, so it could be correct.
The HP-marked barrel is pretty rare, since so many pistols had their barrels replaced.
It's definitely not original, but it's in good enough shape that it could be restored, which would mean spending $2000 to make the pistol worth $1500, maybe, and even then, you shouldn't shoot it much.
EDIT: Lanyard loop mags were long gone by 1918. The correct mags would be two-toned. Original lanyard loop mags are worth some bucks.