gear down speed: early vs. late skymasters

[gevmage]

Not a pilot yet, but I intend to be in a year or so. The Symaster design is interesting to me, so I've purchased a couple of Skymaster "Owner's Manuals" via ebay. I have manuals from the 1966 and 1974 models.

In comparing the two, I ran across a curious difference. Looking at the airspeed limitation charts in both books (page 4-2), the speeds line up pretty consistently, with the '74 model speeds slightly higher. There's one big difference, though, in the gear down speed. The '66 model lists:
Gear operation........160 mph
Gear Extended.........228 mph
(228 mph is also the "never exceed speed"). The '74 model says:
Maximum Speed, Landing Gear Extended....140 mph

That seems odd to me. is there some big fundamental difference between the extended landing gears of the two models that the safe operating speed with the gear down is hugely different? Or is just a change in regulations when the two models were certified, like the fact that the early turbo models were certified up to 28,000-odd feet, but the later ones were only up to 20,000?

Any thoughts?