Quotes are also used as symbols. ' is foot or feet, " is inch or inches, so 5" would be five inch. He's talking about the 5"25, AKA five inch stubby. GAR class on up has an option for fore and aft 5" deck guns, I just checked in the stock game and that option is already available in 41 for 2000 renown points.
Did a tute once, let's see;
http://www.subsim.com/radioroom/showthread.php?t=211849
Post 22 is where we figured it out, edit the (subtype) .upc file for the upgrade pack. In the \Data\UPCData\UPCUnitsData\UpgradePacks.upc is;
[UpgradePack 25]
ID= Upack5in25calDGDouble
NameDisplayable= 5" 25 cal Deck Guns (Bow and Stern)
FunctionalType= UpFTypeWeaponCannon
UpgradePackSlotType= USDeckGunAdv
Type=NULL
UnitUpgradePackIntervalOptionCurrent= NULL, NULL, 0
UnitUpgradePackIntervalOptions1= 1941-01-01, NULL, 2000
IDLinkUpgradePackElements= 5in25calUS, 5in25calUS}
Change
UnitUpgradePackIntervalOptions1= 1941-01-01, NULL, 2000 to
UnitUpgradePackIntervalOptions1= 1941-01-01, NULL, NULL and you can get it for free.