Hi Hans,
Bert Pol’s pulley have no offset. He also tells me that the one’s he sells are the only one’s in the part book.
Your comment got me thinking so I had a quick look through some 2 3/4hp parts books I have that are from different years and found something that might point toward there having been two different plain pulleys avail. for 2 3/4hp two speeds that wouldn't be obvious if you only looked at the common 1925 edition of the parts book.
I agree with Bert that the pulley he sells is the one shown in the picture in the parts book as the picture didn't change from at least 1922 onwards (I don't have any parts books between 1916 and 1922. The 1916 picture of the plain pulley for the two speed is different to the 1922 one but the pulley appears to be the same 781D part).
Gear Box Group - (Two Speed)
1916Picture and description - 781D Pulley (plain)
1922Picture and description - 781D Pulley (plain)
1924Picture shows part 781D
description;
781D Pulley (plain) 1912-20
and Douglas added;
748D Pulley (plain) 1921-24
(note: no picture of 748D)
1925Picture shows part 781D
description;
748D Pulley (plain) 1912-25
781D Pulley (plain) 1912-20 See 748D
1927(list only - no pictures)
description;
748D Pulley (plain) 1912-25
no part 781D listed
So did Douglas have an additional plain pulley, 748D from 1920 onwards and discontinue 781D in 1925 or was 781D the same as 748D? My guess was that there were two different pulleys and Douglas rationalised their spare parts around 1925 at the end of the 2 3/4hp run when they were just trying to get rid of all the outdated models out the door as fast as possible(?). Not sure at all tho'.
I will measure the offset difference between the "748D" one I have (if that is what it is) and a 781D pulley and see if it matches what you need.
I guess the above information probably doesn't help you too much but hopefully you can find an original one to suit your needs rather than modify the 781D one you have.
EDIT: From Ian's post (which came through just as I added this post) it appears that Stephen Sellar's pulleys were possibly the "748D" pattern?? I have just found out from an old post that sadly Stephen is no longer with us.
Cheers
Hutch