The 39C and 12C have their handles fixed to the baseplate. You would not be able to use a custom handle on them. I've seen that Elite sells 2 piece Merkur razors with custom handles, but I'm betting that they are still a 2-piece design, with the original handles being replaced by the custom fixed handles.
If you are looking to get a razor with one of the interchangeable custom handles, you need to look at 3-piece razors such as Merkur's 1904 classic, the Goodfella, or vintage Gillettes such as the Old Style, the New Improved, the NEW, or the Tech.
[LIST][*][I][SIZE="1"]Replated Tech W/ iKon Gripper handle, 1969 Gillette Super Adjustable Long Handle, Goodfella Satin Black w/ BRW Ebony Handle, Replated NEW head W/ BRW 3.75 inch Barberpole handle, 1918 Gillette Ball-End Old Type, Merkur 12C, 38C, 39C Slant[/SIZE][*] [SIZE="1"][I]CROMA, Shark, Derby or IP Blades[/I][/SIZE][*] [SIZE="1"][I]2009 Semogue Horn Handle Silvertip Brush, 2008 B&B Limited Edition Brush, Series 2 B&B Essential, or Tweezerman[/I][/SIZE][*] [SIZE="1"][I]Proraso Pre/Post cream, Shaving Soap/Cream varies[/I][/SIZE][*] [SIZE="1"][I]TGR or Woody's ASB[/I][/SIZE][/I][/LIST]