hi there, a differen type of HT low mount and history??

[Promo]

Compare it with pictures of these two sniper rifles:
Milsurps Knowledge Library - 1916/1945 No.1 MkIII* H.T. Sniper Rifle (Medium Bracket)
Milsurps Knowledge Library - 1917/1945 No.1 MkIII* H.T. Sniper Rifle (Short Bracket)