6pr 7cwt case with unusual stencil ?

[Gspragge]

I picked up this typical common 6pr 7cwt case this weekend.
I didn't pay much attention to it until this evening. The headstamp is the usual
but I have never come across the stenciled black band around the base and the letters HV in two places.
I presume this means High Velocity, so a non standard load ? or for the Disgarding Sabot projectile ?
The base was thick with green and plaster bits oddly -

 

[ThomasP]

There were two different charges for the standard 6.291 lb AP projectile.

The original charge gave a MV of ~2650 ft/sec when fired out of the 43 caliber barrel, while the HV load gave a MV of ~2800 ft/sec.

The original charge gave a MV of ~2800 ft/sec when fired out of the 50 caliber barrel, while the HV load gave a MV of ~2950 ft/sec.