Beneilli M1 S90 Forend Compatibility

[dohboy]

I have an HK branded Benelli M1S90 with an 18" barrel. I recently obtained a 26" M1S90 vented barrel and forend to use for 3GN. The forend from my 18" is different than the 26" forend. The opening at the rear of the 18" forend is larger and allows it to fit over the mag tube where the opening on the 26" forend is smaller and will not fit over the tube. I am able to install the 26" barrel on my HK M1S90 as long as I use my 18" forend. If I use this combo (18" forend with 26" vent barrel), am I good to go?

I am by no means a Benelli expert or historian, so can someone explain the differences with the forends?

The picture with the receiver and forend are from my 18" HK M1S90. The other pictures show the differences between the forends (18" on the left in both pics). Apologies for the poor pic quality. Thanks in advance.