First my apologies for not having the facts nailed down tight, but I have been unable to find as many references as I would like to help accurately date the introduction of the various models. I will edit this page as I find more information. My thanks to Keith Walker for pointing out that later Paxettes were marked with serial numbers.
The first Paxettes made by Braun were simple viewfinder cameras, with the Paxette dating from 1950, the Paxette II being introduced slightly later. My assumption here is supported by the the fact that the earliest instruction book I can find for what later became known as the Paxette I model, is entitled "Paxette Instruction Book", not "Paxette I Instruction book" as might be expected if both models appeared at once, and this instruction book appears to be from 1952.
(An early Paxette manual on Mike Butkus's website.)
By 1954 both models were being advertised in the British Journal Photographic Almanac. The cameras bear no identifying model number, but they can be easily distinguished from each other because the Paxette has a fixed lens, while the lens is interchangeable on the Paxette II.
This first generation have flat top covers with the accessory shoe being integral to the casting. Both cameras have a knob to advance the film, both cameras are fitted with an extinction exposure meter, and both cameras have a flash-synchronised Prontor S shutter with speeds from 1 to 1/300 second.
The instruction book makes mention of a Paxette fitted with an f/3.5 lens in a four-speed Vario shutter. I'd never heard of this model, but they do really exist. I finally found a picture of one on the internet here.
A Paxette with Vario shutter
The example I have has the pressure plate fixed to the camera back, and lacks any serial number on the body.
Features added in later Paxette models included a rapid-wind film advance lever to replace the advance knob, an uncoupled rangefinder in place of the extinction meter, and a change in the shape of the top cover casting.
The top cover changed shape twice, one version is flat like the original, except the shoe is no longer integral, but is made of pressed steel. The top of the shoe now sits a millimetre or so above the casting. Another change saw the top cover casting change to one with a raised centre section where the accessory shoe is fitted.
The change from the integral accessory shoe may have been prompted by concerns around possible damage to the casting if something oversize was forced into the shoe. A separate shoe of pressed metal in the traditional form has considerable 'give', and can take a great deal of abuse before fracturing.
The change to the top casting with a raised centre section may have been driven by a need for more room 'under the hood' to fit in a rangefinder.
The change to a rapid-wind lever is a different matter, this was probably not something changed to overcome an engineering limitation, but was done simply to add value to the product by making it more convenient for the consumer to use.
So in what order did these changes happen? Well, Braun didn't engrave serial numbers on the earliest cameras, that came later, so I can't use them as a guide, but I'll make a stab at sorting it out now. From looking at pictures in catalogues, and the cameras that I have, it looks to me that the first change was the move to the separate accessory shoe. If the illustrations in the BJPA of 1954 are to be relied on, then the change to a pressed accessory shoe came in 1954 or later.I have a Paxette IIM in my collection with a film advance knob, instead of a rapid-wind lever, so the Paxette IIM, with it's uncoupled rangefinder, and the new top cover with raised central section must predate the move to the rapid-wind film advance. As there was no mention of this model in advertisement in the 1954 BJPA, I assume here that the IIM was introduced in 1954 or later.
You can see in the picture that the style of both the film advance and rewind knobs has changed from the earlier cameras, and the rewind knob now has a film reminder dial in the centre.
The Isco Westar lens on this camera lacks the cam to couple to a rangefinder, and so would have not been suitable for a later Super Paxette, but that presents no problem with a Paxette II or IIM.
This example has the pressure plate fixed to the body hinged at the top, and lacks a serial number on the body.
Another IIM I have, but this time with an advance lever, also has the pressure plate fixed to the body at the top, and also lacks a serial number.
I also have this Paxette I with flat top cover, and separate, pressed accessory shoe, with a rapid-wind lever, so the Paxette I with flat top cover must have continued in production after the introduction of the Paxette IIM. For some time there must have been Paxette I models with flat top covers, and other Paxette models with raised tops in production at the same time.
This camera also has the pressure plate fixed to the body at the top, and lacks a serial number.