Linking <- data.frame(
  ID = c(round((runif(20, min=10000, max=99999)), digits=0), rep(NA,10)),
  PSU = c(paste("A", round((runif(20, min=10000, max=99999)), digits = 0), sep = ''), rep(NA,10)),
  qtr = c(rep(1:10, 2), rep(NA,10)),
  date = rep("13/04/56", 30),
  Direct = rep(c('D','M','U','U','M'), 6),
  stringsAsFactors = F)

Linking$Key <- paste(Linking$ID, Linking$PSU, Linking$qtr, sep='_')
Linking$Key[c(21:30)] <- c("87654_A15467_1", "45623_A23456_2", "67891_A12345_4", "65346_A23987_7",
                       "E3456782_A456321_6", "E3421986_A34564_8", "E9859873_A123456_9", "E3452_A12345_6", "R765498765_A455634_2", "54678_A12345_5")

test <- filter(Linking, is.na(ID)) %>%
 select(Key)
 test2 <- data.frame(do.call(rbind, strsplit(test$Key, "_")), test$Key)
 names(test2) <- c("ID", "PSU", "qtr", "Key")

tmp <- data.frame(do.call(rbind, strsplit(Linking$Key, "_")), Linking$Key)
names(tmp) <- names(Linking)
tmp