The little difference
The difference between the 1609 and the 1648 is the position of the waste pipe exit. The waste pipe exit of 1609 is below the floor while the 1648 has one above the floor at the rear. The vacuum action stays the same.

(image shows VT 1609)

• It clears the china toilet bowl in an instant while fragmenting the waste.
  
  
• Durable design from handcrafted china: Extremely durable and
  scratch-resistant for thorough cleaning.
  
  
• Seating in standard height like at home for comfortable access.
  
  
• Household sized seat.
  
  
• Light weight.
  
  
• Effective rim flush for perfect cleaning.
  
  
• Self-cleaning flush ball with Teflon seal, locks out any odour.