H&C & Circle C Stock

I'm just guessing, but 208, 209, 238, and 239 aren't used because they are a number greater than 7 and therefore can't be recognised by the Programme Machine. And are the H&C 700-703 used for S7 Testing?

Ok thanks. When all the Programme machines are got rid of, will TfL still omit numbers including 8s and 9s, because of tradition, or would the Management try to keep the tube "modern"?

Well I guess that all depends upon what is kept and what is thrown out. Train numbers are octal, This was the digital technology of the time when programme machines were introduced circa 1960 or thereabouts. Early signalling computers installed in the 1980s were also octal and AFAIK some if not all remain so although they are accessed via PCs which use hexadecimal and translate to octal.
I expect that all such technology will be swept away in time as lines are resignalled. There's nothing at all wrong with octal except that it is 'old hat' these days with 64 bit processing becoming standard. Much of LU still relies upon octal technology including systems that were designed around UARTs in the 1960s long before the invention of the CPU chip.

Most LU technology has stood the test of time, some of the technology is still in use more than 100 years after it was designed, it was built to last and last it has. I'm not sure that the same will one day be said of systems installed today although I have an open mind on that.

Perhaps if the whole thing went over to hexdecimal, the full set of characters could be used up to 'F', and with the vast amount of new numbers possible every train on each line numbered into one over all series? Does the tube require more than 4,096 train identifiers per day?