What does an 84 silver mark mean if its the only mark on the piece?

While "84" is indeed a number used on Russian silver, it is generally NOT a percentage of silver, it refers to 84 ZOLOTNIKI, a Russian measurement in which there are 96 zolotniki to FINE (100%) SILVER.

Therefore, 84/96 parts is .875 fineness silver.

However, perhaps 84 might represent percentage in some other country.