408:

I agree with your assessment 100%. Even forgetting the DG issue, I want to be able to drop a round in the chamber, tho in practice I normally just jam rounds in the mag. Truth is, even for those who feed from mag only, as you say, inadvertently dropping a round into the chamber frequently leaves the round stuck in the chamber.

.470:

I have Gun Digest '87, '90, '91 and '93.

'87; no mention of MKII.

'90; MKII & M77 rifles listed. No mention of Controlled Feed in the features list of MKII guns that includes "New trigger and three-position safety". Regarding MKII, says "Introduced 1989".

'91; Ditto.

'93; Maybe here is the source of the confusion... "Controlled Feed" listed for the MKII Magnum rifle only. I wonder if CF was introduced first to the Magnum action line and then to the standard action lengths later? So maybe Ruger and you are both correct. Intro'd in '92/'93 "era" but first in the Mag action, later {when???} in standard length actions.