The M39 has a barrel bushing that must be rotated a bit (I forget which way) in order to remove the barrel. It also doesn't have (AFAIK - mine doesn't) a right-side safety, and the rear sight is bayoneted (I guess that's the right word) onto the slide. Essentially, there's a flat strip of metal forward of the rear sight that needs to be lifted, and then rotated, to remove the sight. The magazine disconnect may go flying if you do that.

I bought mine new sometime before 1975, carried it for a number of years, and had it on the range last week. I switched to 1911's (and M&P40s) a while back, and now find the "backwards" safety a hindrance, so it's a range gun now.... No regrets, though.

(The front sight on mine is integral to the slide, too. The pic in the PDF shows one that's removable.)

Regards,

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Stu.