This is quite an unusual find, it has an ex loco rear and at first glance is obviously a replacement plate made to replace one lost from a 1925 built loco, note the type face is Burn Co block type as introduced by the LMS and later by BR for use on plates for locos of LMS design built by BR (but we'll come to that later as its not the only type face used for that purpose!)
This was a bit of a mystery at first until I noticed a very faint cast in mark to the left of the 1925, which after localised cleaning turned out to say '1120'. This same number appears in the same place on the Horwich plates fitted to LMS engine built under BR in the same spot. Given the large size of the plate I believe the master pattern was quickly re-lettered and used to make this replacement.
 
Basic dimensions in inches- 10 & 11/16th by 6 & 1/8th
Pattern Rarity- Very Rare, 1 seen.
Special Features- Horwich pattern number on front, Burn Co letters, this style of lettering appears only to have been used on LMS designed locomotives built by BR.