It was designated by Sukhoi as the T10V-1 and was the first variant of what later was designated as Fullback.
The rightmost Fullback in the lineup is the latest configuration - with the boarding steps permanently attached to the nosewheel leg.
The one in the middle is an interim configuration.
All Su-34s after bort 42 had the twin mainwheels, shorter fins, extended tailbooms etc.
AFAIK, the development a/c are numbered 42 to 47.
Borts 01 & 02 are the ones recently delivered to the 4th TsBP i PLS at Lipetsk