From that link (See highlighted on the attached) in the link you posted Phil, it looks to me like RPO 274 was just for 6.00-16 6 ply tires mounted on the same 16x4 wheels that the early AD trucks used. GM went to 16x4.5 wheels on AD's around 50 or 51. I'm thinking those wider 16x6 wheels are later than 1955.
[on edit] I checked a few later year spec sheets and Chevy offered optional 6.50-16 tires at least thru 1978 but they were mounted on 16x5 wheels. I haven't found any mention of 16x6 wheels. As I mentioned earlier, other manufacturers used the 6 on 5.5 lug pattern.
