I'm no lawyer, but I just read HB294 full text and summary and from what I saw, it appears that switchblades are legal as long as you're within Federal law etc. (1), and actually removes some of the restrictions such as curio/relic(2) and non-functioning(3).
(Perhaps someone with greater legal expertise woud be so kind as to review my humble interpretation's accuracy?)
Excerpts:
(1) Summary 4) states
"...A person will not commit a crime if he or she possesses, manufactures, transports, repairs, or sells ..... a switchblade knife.... if he or she conforms with federal law."
(2) Sect 571.020 lines 28-29 remove restriction
"[..solely as a curio, ornament, or keepsake,..]"
(3) Sect 571.020 lines 30-31 remove restriction
"[...must be in such a nonfunctioning condition that it cannot readily be made operable...]"
Links:
Missouri HB294
Summary
Full Text pdf with line #'s