It's not a "Sergeants" pouch, it's the pouch for small articles.
The 1918 Tables of Organization and Equipment (TO&E) lists an Infantry regiment as having 239 rifles. 235 bayonets. What of the other 4? Scopes. 256 haversacks. 31 pouches for small articles. What we can gain from that is:
1) Not everyone had a haversack.
2) The pouches weren't just carried on haversacks.
Number 2 there might not be obvious. Not everyone had an intrenching tool so there are enough haversacks, without intrenching tools, for all of the pouches for small articles to be hung from haversacks right? Except it wasn't hung from the intrenching tool tab on the haversack, it was hung from the bayonet tab. Thus the long strap as it was mounted on the bayonet strap and then ran over the right shoulder. So states Training Regulation 50-80. "The Infantry Pack."
Four items are listed as being carried in it. Could be more but four are listed:
1) Shoe brush.
2) Clothes brush.
3) Housewife.
4) Rifle screwdriver
Condoms didn't make the list.