Sorry for any confusion.
I'm using Dillon's terms for their parts, and they aren't clear even if you look at their catalog or site.
In comparing the 550b to the Pro2000, there is no station available for the expander die on the 550b. While it may be possible to "jury rig" a 550b to do something with a mix of parts from different mfgrs, if you follow their advice, you use a Dillon powder die at Station 2.
Inside this die you have to insert a powder funnel, of the correct size for the caliber you are reloading, and then install the powder measure on top of everything. This funnel also serves as the expander die, and so some call this setup "powder thru expander".
As the critical adjustments for "bell" and powder measure actuation are controlled by the die adjustment to the toolhead, you can buy a powder die for each toolhead and quickly move the powder measure between them without having to adjust that. All that remains is adjusting the powder measure for the throw you want. (And possibly changing powder bars, if you want more/less than the present bar will handle...)
Using the two calibers you mentioned, you could share a powder measure between .44 mag and .45 ACP toolheads by removing the measure from one powder die and attaching it to the other. As the powder measure only comes with one powder die, you have to buy another one ($15) for the other toolhead to take advantage of this. The powder funnel belling adjustments stay intact and the powder dies stay attached to the toolheads.
(This is a "compromise" to full-out quick change, which is having a dedicated powder measure, with the correct bar, for each toolhead.)
Edit - The last time I checked, Dillon shows a picture of a spare toolhead with a spare powder die installed. This is a broad hint... You have to order the two parts separately, unless you order some "deluxe" conversion kit, which I think includes a powder measure. After you've messed with a 550b for a while, this is a little clearer. But I'm not sure it is clear on first examination, from Dillon's information.
.30-06 Springfield: 100 yrs + and still going strong
Last edited by dmazur; March 11, 2012 at 02:34 AM.