The No 5, which was the same design as Victor's Orthophonic soundbox of 1925 , appeared on re-entrant models at the end of 1927. It proved totally unsatisfactory and was soon replaced by the 5a. Some of it's features were retained by the 5a, eg, the "spider" on the diaphragm and the diaphragm patterning but the pivot system for the needlebar reverted to the system of the No. 4. However despite the changes the 5a still demonstrated short comings in the form of a "buzz" which led to the instruction to "close lid whilst playing" on models like the Model 104 and Model 163.
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Nickel plated version seems to have been used on cabinet and table models from 1927. A chrome plated version was used on portables from around 1934. Note the semi circular holes below the screws on the chrome version. Occasionally I have also found a washer under the front cover plate behind each screw. No idea what the purpose of these was.
Related topic: Making a replacement No.5 insulator
Finally the 5a was replaced by the 5b version in the late 1930's, probably 1939.The major difference between the 5a and 5b seems to be a change in the pattern on the diaphragm. I also believe the pot metal was of better quality than the earlier 5a
Related Topic: Making a replacement No.5 insulator
The No. 16 soundbox was developed initially for the new 102 portables but this eventually gave way to a modified 5a in 1934. Rather harsh in sound the 16 was impossible to repair as its font and back were permanently joined together. It carried a screen presumably to cut down buzzing on the then new 102 which had to be played with an open lid.
Note: The soundbox listing is not comprehensive. It merely relates to the soundboxes on my collection of gramophones.