Looking at old photos there seems to be three types of pillion seats fitted to the 3HW,the early long type (Mansfield),the mid war type (Lycett),and the late war short type (Mansfield),below is the early long type. 
Photo below is the late war short type. 
The the early long type and the late short type together,they are made by Mansfield. 
Below a photo of 3HWs at Littlewoods Crosby factory,it shows five motorcycles and how they were packed into the crates in the back ground,two types of pillion are fitted the long early type and the Lycett type. 
Below is the Lycett type fitted to 1943 and 1944,the short type looks like they were fitted late 44 and 1945. 
Below a rear view of the Lycett mid war type. 
The reason for the shorter saddle was not just for the sake of economy, it was mainly because some riders were injuring their spine on rough off road riding. Another answer was to fit a soft pad at the front like on this Ariel W/NG. Triumph also did this on some 3HW's. Ron
email (option): ronpier@talk21.com
Hi Jan,
It looks like the Lycetts pillion seat with the front springs is 'early war'
and the one with the rear springs is 'late war'
I'm not sure when they changed from front to rear springs, it could be 1942 when other specs also changed.
Regards,
Sven
email (option): snvosselman@gmail.com
And I might be wrong about the front hump pillion saddle on 3HW's. I was relying on someone else's research but I haven't proven it yet
I think as shown they simply fitted a shorter saddle as did BSA. Ron
email (option): ronpier@talk21.com
October 1943 photo,the pillion seat is the Lycett type,these may have been fitted well into 1944. 
The above photo was taken at Meriden in October 1943,my own 3HW could be among them,frame no TL 39674 under the seat no T.E.C.10.43,the photo below could be earlier,it shows the Lycett type pillion,got a photo of the late type but cant find it at the moment. 
An advert from a 1948 MOTOR-CYCLE,a similar Mansfield pillion seat as fitted to the 3HW,but could be post war made,or ex WD stock like some of the other spares in the advert. 
An update on Mansfield pillion seats fitted to Triumph 3HWs.
Photo below showing fixing points for the mudguard,both long type and later 1945 short type.
Photo below show leather straps that hold the cover on there are 4 of them.
Photo below a Mansfield seat (not Triumph) with traces of service brown.
Below the Mansfield seat cover,leather straps have gone dry over many years and broken off.
Photo below,Mansfield pillion seat on a Triumph 3HW in the Far East 1945.
Great info Steve. Here's the Mansfield I'm restoring for mine:relaxed: Ron
email (option): ronpier@talk21.com