diff --git a/src/Utils/Print/Letters.hs b/src/Utils/Print/Letters.hs
index 0718a294b..f645772c9 100644
--- a/src/Utils/Print/Letters.hs
+++ b/src/Utils/Print/Letters.hs
@@ -177,17 +177,30 @@ data PrintJobIdentification = PrintJobIdentification
 --   this is printed in white on white at the exact same position on the page
 --   Note: that all letters to the same UUID within 24h are collated in one envelope
 --   Example: 9ad8de3f-0a7e-ede5-bd8b-6d0ed85c1049-f___a4pin___230322-10___lms-stuvwxyz
-mkApcIdent :: CryptoUUIDUser -> Char -> LetterKind -> Text -> Text -> Text
+mkApcIdent :: CryptoUUIDUser -> (Int, Char) -> LetterKind -> Text -> Text -> Text
 mkApcIdent uuid envelope lk tnow apcAck = Text.filter apcAcceptedChars $ Text.intercalate apcIdentSeparator 
-      [ ensureLength 38 $ tshow (ciphertext uuid) <> Text.cons '-' (Text.singleton envelope)
+      [ ensureLength 38 $ tshow (ciphertext uuid) <> mkEnvelope envelope
       , ensureLength  5 $ paperKind lk
       , ensureLength  9   tnow
       , Text.take    32   apcAck -- length of last part may be arbitrary, but more than 32 symbols do not fit into the line
       ]
-  where 
+  where
     ensureLength :: Int -> Text -> Text
     ensureLength n = Text.take n . Text.justifyLeft n 'x'
 
+mkEnvelope :: (Int, Char) -> Text 
+mkEnvelope (i,c) = Text.cons (pfx $ i `mod` 63) $ Text.singleton sfx
+  where 
+    pfx n
+      | n <=  0   = Text.cons '-' $ Text.singleton c
+      | n <= 10   = Text.cons (chr $ ord '0' + n `mod` 10) 
+      | n <= 36   = Text.cons (chr $ ord 'a' + n - 11)
+      | n <= 62   = Text.cons (chr $ ord 'A' + n - 37)
+      | otherwise = Text.cons '-' $ Text.singleton c
+    sfx 
+      | n <= 62   = c
+      | otherwise = Char.toUpper c
+
 formatApcIdentTime :: (HasLocalTime t, MonadHandler m) => t -> m Text
 formatApcIdentTime = formatTime' "%y%m%d-%H"
 
@@ -245,7 +258,10 @@ letterApcIdent :: (MDLetter l, MonadHandler m) => l -> CryptoUUIDUser -> UTCTime
 letterApcIdent l uuid now = do
   -- now  <- liftIO getCurrentTime
   tnow <- formatApcIdentTime now
-  return $ mkApcIdent uuid (getLetterEnvelope l) (getLetterKind l) tnow (pjiApcAcknowledge $ getPJId l)
+  let startEnvelope = (0, getLetterEnvelope l)
+      mkApcEnv env = mkApcIdent uuid env (getLetterKind l) tnow (pjiApcAcknowledge $ getPJId l)
+  -- TODO: turn this into a loop increasing the number for each 6 unacknowledged letters to the first part of the ident only      
+  return $ mkApcEnv startEnvelope
 
 letterFileName :: (MDLetter l) => l -> FilePath
 letterFileName = Text.unpack . (<> ".pdf") . text2asciiAlphaNum . pjiFileName . getPJId