THe AMT 49 Merc NEVER had a steel axle thru the engine. It was ALWAYS released with STEERABLE front wheels since it's original intoduction. The 49 and 50 Fords had the thru the engine axles.
I think George is right- I've had several versions of all three kits over the years- the '49 Ford still had a metal front axle at least up to the 'Crusin' USA' issue, the '50 Ford still had one in the most recent reissue, but the Merc has always had a poseable setup with plastic kingpins up front. Oddly, even though the 1:1 cars were mechanically identical, all three kits were done from their own separate tooling.