As far as I am aware, no true Harry Pedler clarinets (1919-1930) were ever adorned with a serial number, and some were not even engraved at all--even with his name, during the earliest years. The standing exception is that BBb models may have been serialized early in their production. Until disproven, it is a cautious measure to operate under the calculated assumption that the serial number process began across the model spectrum when Martin Band Instrument Company took the reins in 1930.
I believe this to be at least partially incorrect. I've recorded a number of serialized Pedler metal clarinets that have every appearance of being made prior to the Martin era. The Premiere model, which was introduced in 1928, begins about where expected and I've never seen one (nor any other metal Pedler) without a serial. It's quite plausible that serialization began in the latter half of the 1920s rather than upon the Martin purchase.
I've noticed that metal Pedlers after a certain point have beveled toneholes. I'd have to surmise that those were influenced by Martin saxophones. Other notes I have:
The Pedler metal bass was introduced in March 1930 at the Music Supervisors' National Conference. This was the same month that Martin purchased Pedler. Multiple sources state that Pedler was operated as a separate entity after the purchase.
The Music Trades Review reported in April 1931 that the Pedlers had resigned from the company.
I've made note of a Premier (not Premiere) serial no. P8654 that was sold on 6-20-1942 for $121.