The Panasonic remote control uses pulse distance encoding where pulse widths for both logic 1 and 0 are the same: 2T, but the periods are not: 4T for logic zero and 8T for logic 1. Thus a logic 0 is high for 2T and low for 2T while a logic 1 is high for 2T and low for 6T. The start pulse is 8T high and 8T low. From the logic analyzer measurements T for the particular unit I have is anywhere from 435 to 440µs. (Bear in mind that at the receiver end everything is inverted--high is low, and low is high, with the output of the IR receiver normally high when no transmission is being received.)
The remote control sends out the least significant byte (LSB) is first. And for every byte the least significant bits (LSb) are sent out first. A start pulse and four bytes constitute one packet. The LSB is the command code. This code is sent again (without inversion) as the second byte. The third byte is the address code. This is transmitted again and is the fourth and most significant byte.
With the exception of three, keeping the buttons depressed does not continually transmit a signal. Instead exactly three packets are sent without pause (i.e., a start pulse immediately follows after the last bit of the packet has been transmitted). The series of three packets is then capped with an end pulse which is equivalent to a start pulse and 2T high.
Holding down the up arrow, down arrow, or fan speed buttons will continually send the packet (with no end pulse). Panasonic must've made a mistake with the fan speed button since it need not send the packet continuously. Testing shows that even if the fan speed button on the remote control is kept depressed the speed of the blower is toggled only once per button push (it has to be released and pressed again to change fan speed).
From the looks of it the numbers at the back of the control are stamped after molding. Another Panasonic air conditioner remote has the same "model number" A75C2454 but has "batch number" 9225 instead of the 91X4 on this unit.
I tested all the buttons on the remote control and collected the following command and address codes.