129.4200 does not exist as an aviation frequency (In actual fact, if it did exist, it would be just 129.420 as only a maximum of three decimal places are used for civil aviation R/T frequencies). The used frequencies are spaced at 8.3333333333 recurring kHz (0.0083333333333 recurring MHz), referred to 8.33 kHz for short. This gives 12 available channels per 0.1 MHz. As it is completely ridiculous to say ".........point 3333333 recurring" over the R/T, the frequencies are rounded to the nearest 5 kHz for ease and to reduce the chances of confusion. So referring to the question asked above, the adjacent channels would be:
Actual frequency R/T name
129.400 129.4 (the double zero is dropped for ease)
129.40833333 129.410
129.41666666 129.415
129.425 129.425
129.43333333 129.430
129.44166666 129.440
129.450 129.450
.........and so on. This is why 129.420 does not exist as an aviation R/T channel. Hope this answers your question.
T6
PS Can't get this to display in nice columns as it does when I type it in, but hope you get what I'm describing.