RE: interactive codes "p" and "P" - bug or mistake

[Drew Adams]

> IMO the code "p" sends the numerical argument, while "P" the Prefix 
> value introduced with C-u

Correct.  And BTW, the "numerical argument" is nothing more than
`prefix-numeric-value' applied to the raw prefix arg value (the "prefix value
introduced with C-u").

> So if I do M-NUMBER, "P" should send nothing.

No.  There is _always_ a raw prefix-argument value, even if you use none of
`M-NUMBER', `C-NUMBER', `M--', or `C-u'.  If you use none of those then the raw
prefix argument value is nil.

If you use `M-NUMBER' or `C-NUMBER' or `C-u NUMBER' then the raw value is
NUMBER.  This is mentioned in the doc we pointed you to:

 "M-3   M-x display-prefix -| 3  ; (Same as `C-u 3'.)"
 "M-- 7 M-x display-prefix -| -7 ; (Same as `C-u -7'.)"

> whilst C-u M-command
> both should send their defaults, ie "p" 1 and "P" (4)

I don't understand you.  Why `M-command'?
But yes, `C-u' by itself gives 1 for "p" and (4) for "P".