Ok..... um, your last message doesn't make much sense. Proofread before you post please.
Yes, I'm 100% sure that the 551 is the correct blower for your machine. I don't know why a cub dealer wouldn't know the info, and I don't know who you are talking to.
Look it up for yourself. Go here:
Cub Cadet
Now, the page I linked you too already has the 1863 pulled up. There is a serial number break on the 1863. You can choose whichever one applies to your machine, but I'll save you time. It doesn't matter because the SN isn't important in this instance. So, go over to the right side and click on the "select assembly" button. Then when the pop-up comes out, choose "attachments". Then scroll down toward the bottom. You will see the 551 listed as an attachment for your machine.
We won't steer you wrong here at OCC. We are the "premium" Cub Cadet site!
On a side note: The 450 is not for that machine, but the 451 will also work. Only difference is in appearance between the 451 and the 551. Functionality/application is the same.