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the -S denotes it's a programmable radio. the front of the radio will have a MX310, 320, 330, 340, 350, or 360. that denotes the size of the case. I have seen 340, 350, and 360 that are programmable. what you really need to look for is the model number on the rear.. it should look something like. H43SXU3140
H= Hand HEld
4 = High Power in a hand held ( 2 or 1 Would be low power in a hand held)
3 = VHF (4 is UHF, 5 is 800, 1 is Low Band)
SXU = Programmable, Enxrypted (AAU is Crystal, SSU is Programmable, no encryption
31 = CTCSS (51 is DPL, 11 is Carrier Sq.
Now I am not an expert on the radio, but I do have a working knoledge of them. PM me if you have more questions.
To add to what DFC2 told you, the case length varies as the features area added, for example:
1. More frequencies in a crystal radio
2. More PL modules in a PL-selectable module radio
3. Encryption module in an encrypted radio
4. More "anything" that requires more physical space.
The Motorola MX service manuals have all of this variable data shown in extreme detail if you can get hold of one.
The smallest MX you're likely to find is a single-channel radio which has only two crystal modules, in UHF the channel set and offset modules, and in VHF the transmit and receive modules. This would be a 320 which is a really nice short radio but it's also short on features. A single PL capability doesn't usually add to the case length, only when you get to mulltiple PL radios.