How to add file creation date, user, file name, and file path to header template when creating a new file in VIM / GVIM (v7.4)?

Question

I have a default header file that is loaded by the following command in my ~/.vimrc :

autocmd bufnewfile * so /home/username/.vim/header.txt

It looks like this:

######################################################
# File name:      
# File path:      
# Purpose:         
# Creation date:  
# Last modified: 
# Created by:      
######################################################

I want to update Last modified: with the date/time when saving (using autocmd BufWritePre). This is working as expected.

I also want to populate File name:, File Path:, Creation date:, and Created by: when a new file is created (using autocmd BufNewFile). This is not working. Nothing is added to the header when a new file is created.

This is my code for updating Last modified: when saving a file. I also added in updating Created by: just to test my pattern match, and this code updates both fields upon file save as expected:

autocmd! BufWritePre * :call s:timestamp()
" to update timestamp when saving if it's in the first 20 lines of a file
function! s:timestamp()
    let pat = '\(\(Last\)\?\s*\([Cc]hanged\?\|[Mm]odified\|[Uu]pdated\?\)\s*:\s*\).*'
    let rep = '\1' . strftime("%a %d %b %Y %I:%M:%S %p %Z")
    call s:subst(1, 20, pat, rep)  
    
    let pat = '\(Created by\?\s*:\s*\).*'
    let rep =  '\1' . $USER
    call s:subst(1, 20, pat, rep)
endfunction

This is my code for updating the File name:, File Path:, Creation date:, and Created by: fields when a new file is created. It does not work; the fields do not get updated and there is no error message.

autocmd! BufNewFile * :call s:creationdata()
function! s:creationdata()
  let pat = '\(\(Creation\)\s*\([Dd]ate\)\s*:\s*\).*'
  let rep =  '\1' . strftime("%a %d %b %Y %I:%M:%S %p %Z")
  call s:subst(1, 20, pat, rep)

  let pat = '\(\(File\)\?\s*\([Pp]ath\)\?\s*:\s*\).*'
  let rep =  '\1' .escape(expand("%:p:h"), '/\'. (&magic ? '&~' : ''))
  call s:subst(1, 20, pat, rep)

  let pat = '\(\(File\)\?\s*\([Nn]ame\)\?\s*:\s*\).*'
  let rep =  '\1' . expand("%")
  call s:subst(1, 20, pat, rep)

  let pat = '\(Created by\?\s*:\s*\).*'
  let rep =  '\1' . $USER
  call s:subst(1, 20, pat, rep)
endfunction

The subst function called by the functions above is defined as:

function! s:subst(start, end, pat, rep)
    let lineno = a:start
    while lineno <= a:end
    let curline = getline(lineno)
    if match(curline, a:pat) != -1
        let newline = substitute( curline, a:pat, a:rep, '' )
        if( newline != curline )
        " Only substitute if we made a change
        "silent! undojoin
        keepjumps call setline(lineno, newline)
        endif
    endif
    let lineno = lineno + 1
    endwhile
endfunction

I can't figure out why the BufNewFile autocmd doesn't work. I've tried several other methods that I found via googling, including:

"  Method 1:  doesn't work; not compatible with vim 7.4?  Gives Error:
"  E16: Invalid rangeError detected while processing BufNewFile Auto commands for "*":
"  E16: Invalid range: 1,10g/File name:.*/s//File name: foo
"  E16: Invalid range: 1,10g/Creation date:.*/s//Creation date: 13-12-2022      
autocmd bufnewfile * exe "1," . 10 . "g/File name:.*/s//File name: " .expand("%")
autocmd bufnewfile * exe "1," . 10 . "g/Creation date:.*/s//Creation date: " .strftime("%d-%m-%Y")

... and:

" Method 2:  only works on saving file, not on creating a new file.  No error message.
autocmd BufNewFile,BufWritePre,FileWritePre * ks|call LastMod()|'s
  fun LastMod()
    if line("$") > 20
      let l = 20
    else
      let l = line("$")
    endif
    exe "1," . l . "g/Last modified:/s/Last modified: .*/Last modified: " .
    \ strftime("%Y %b %d")
    exe "1," . l . "g/Creation date:/s/Creation date: .*/Creation date: " .
    \ strftime("%Y %b %d")
  endfun

It's as if BufNewFile is the wrong autocmd pattern or something. But I've checked the docs and online and I think it's correct.