There were 2 types of speedo fitted to Class 25's.
All the class 24's and 25's up to D7597 (the last of the 25/2's) had the small "original mechanical speedometer", fitted to the outer axle under the drives position at number 1 end.
From D7598 on the control of the locos incorporated electronics to better control, amongst other things, the field diverts, and became 25/3's
Part of this modification was the fitting of an electronic speedo drive (see attached), which was fitted to the inner axle at number 2 end - second mans side.
This makes it easy to identify the difference between the later bodied 25/2 (mechanical) or an 25/3 (electronic) locomotives.
Unless of course you are Heljan and insist on calling a mechanically fitted 25/2 a 25/3!