What you have there is an old practice mute. They are designed for apartment playing where your neighbors cannot stand to hear you practice. They were produced in the late 1930's through the 1960's. It is most definitely an antique (Antique Road Show anyone?) and rather rare to see these days. It does almost completely obliterate the upper frequencies of your sound production, leaving mostly fundamental, this is why is SOUNDS like the bass is louder but duller. Actual decibels are likely reduced in all frequencies. You should sound less loud and less volumous at the same time. It is also a great way to practice your orchestral music while your living partner(s) are trying to sleep; sounding like the rumble of trucks at a distance (for many, a rather soporifying catalyst)!
Hold on to it... you may need it someday!
Patrick
