70 340 duster manual steering column

I assume you mean the lever on the column between the firewall and the coupler. Even floor shift transmission have that in 1970 for the column/key lockout feature. There was linkage that come up from the transmission that attached to that lever. When the transmission was shifted into reverse (I believe, or, was it first?) that would rotate that inner column tube and turn an upper collar to allow the key to be removed. They did away with that in 1971 where they just had a thumb lever on the collar to do the same thing.