That is the difference. VHB29 (Loop) manifolds are 35 mm o.d. and 37 mm for later Guzzi with VHB30s. The i.d. is slightly different as well.
Yes, the 37 mm manifolds could be used with VHB29s if one swapped out the 35 x 40 sleeves for 37 x 40 sleeves. However, last I checked, there were no 37 x 40 (VHB30) sleeves available anywhere. I made a pair for a customer's G5 out of Delrin a few years ago, so that's an option.