diff --git a/ckcc/__init__.py b/ckcc/__init__.py
index 1108766..ee932a7 100644
--- a/ckcc/__init__.py
+++ b/ckcc/__init__.py
@@ -1,6 +1,6 @@
 # (c) Copyright 2021 by Coinkite Inc. This file is covered by license found in COPYING-CC.
 
-__version__ = '1.2.0'
+__version__ = '1.2.1'
 
 __all__ = [ "client", "protocol", "constants" ]
 
diff --git a/ckcc/sigheader.py b/ckcc/sigheader.py
index a39ac43..ff4eb07 100644
--- a/ckcc/sigheader.py
+++ b/ckcc/sigheader.py
@@ -1,5 +1,4 @@
-# Autogen'ed file, don't edit. See bootloader/mk-sigheader.h for original
-
+# (c) Copyright 2018-2022 by Coinkite Inc. This file is covered by license found in COPYING-CC.
 
 # Our simple firmware header.
 # Although called a header, this data is placed into the middle of the binary.
@@ -11,10 +10,9 @@
 # - version_string is for humans only
 # - pubkey_num indicates which pubkey was used for signature
 # - firmware_length, must be:
-# - bigger than minimum length, less than max
-# - 512-byte aligned
+#   - bigger than minimum length, less than max
+#   - 512-byte aligned
 # - bootloader assumes the flash filesystem (FAT FS) follows the firmware.
-# - this C header file is somewhat parsed and used by python signature-adding code
 # - timestamp is YYMMDDHHMMSS0000 in BCD
 
 
@@ -23,23 +21,37 @@ FW_HEADER_OFFSET = (0x4000-FW_HEADER_SIZE)
 
 FW_HEADER_MAGIC = 0xCC001234
 
+# Firmware Image Size
+
 # arbitrary min size
 FW_MIN_LENGTH = (256*1024)
-# absolute max: 1MB flash - 32k for bootloader
+
+# (mk1-3) absolute max size: 1MB flash - 32k for bootloader
 # practical limit for our-protocol USB upgrades: 786432 (or else settings damaged)
 FW_MAX_LENGTH = (0x100000 - 0x8000)
 
+# .. for Mk4: 2Mbytes, less bootrom of 128k.
+FW_MAX_LENGTH_MK4 = (0x200000 - 0x20000)
+
 # Arguments to be used w/ python's struct module.
-FWH_PY_FORMAT = "<I8s8sII36s64s"
-FWH_PY_VALUES = "magic_value timestamp version_string pubkey_num firmware_length future signature"
-FWH_NUM_FUTURE = 9
+FWH_PY_FORMAT = "<I8s8sIIII8s20s64s"
+FWH_PY_VALUES = "magic_value timestamp version_string pubkey_num firmware_length install_flags hw_compat best_ts future signature"
+FWH_NUM_FUTURE = 7
+
+# offset of pubkey number
 FWH_PK_NUM_OFFSET = 20
 
-# There is a copy of the header at this location in RAM, copied by bootloader
-# **after** it has been verified. Cannot write to this area, or you will be reset!
-RAM_HEADER_BASE = 0x10007c20
+# Bits in install_flags
+FWHIF_HIGH_WATER = 0x01
+FWHIF_BEST_TS = 0x02
 
-# Original copy of header, as recorded in flash/firmware file.
-FLASH_HEADER_BASE = 0x0800bf80
+# Bits in hw_compat
+MK_1_OK = 0x01
+MK_2_OK = 0x02
+MK_3_OK = 0x04
+MK_4_OK = 0x08
+# RFU:
+MK_5_OK = 0x10
+MK_6_OK = 0x20
 
 # EOF