"...legal length of a semi barrel?..." Hi. Minimum of 18.5"(470mm). Minimum OAL of 26". That not covered on your PAL course? Mind you, it's usually not an issue for commercial hunting rifles. The U.S. has rules about barrel lengths too.
M742 carbines had 18.5" barrels. Non-restricted. Kind of serious muzzle flash and blast though. Not enough to worry about. The 742 was discontinued in 1980.
No M740 carbine. 22" barrel, only. Discontinued in 1959.
Remington's site has limited history by model on their site under 'Products'. Gives manufacturing dates, approximate numbers made(you'd think they'd know how many they made), chamberings, grades/types offered, variations and sometimes S/N blocks by year. No barrel lengths for any of 'em.