Why the two different bases ? (Short and tall)
The trays are also available separately, for those who might want to install them on something custom or on their swing-away tire carrier or whatever. The tall tray is bolt-compatible with the original Blitz carriers, so it could be bolted directly to the side of a Jeep, for example, and it's also compatible with the position of the strap slots, so any straps that fit a Blitz will fit the tall tray. The short tray is another option for people who prefer or need that style for their custom application. As long as the two trays are being manufactured, it made sense to also offer the choice to people buying either the side-mount or the spare-mount carriers.
Both trays are also compatible with jerry, NATO and Sceptre style cans, and two Rotopax containers can also be carried in them.
The trays are also a much heavier gauge steel than the original Blitz trays, so they're much stronger and more resistant to cans vibrating and rattling around than the original trays.