You are running into the effects of changes in January: https://dev.openwrt.org/changeset/29686
(Bug tracker server is currently down, but that is the change made then.)
Kernel version/name includes now a hash checksum of config options, and the kernel modifications will depend on that checksum as it is part of the kernel version/name. The goal is to ensure that the kmods to be installed have been compiled using exactly the same options as the main kernel.
That will decrease the possibility of installing kernel modules from trunk snapshot directory for older openwrt installations, as the default kernel config changes every now and then.
To be able to reproduce the same checksum, you would need exactly the same kernel options in your .config that have been used when compiling the main kernel. (Just having the same source code revision is not enough.)
In practice, if you have the same source code revision and you have vanilla kernel compile options, there should be no major risk in overriding the checksum. Most likely you will never be able to reproduce exactly the same kernel options, as the options in your .config have already been modified while compiling later source code revisions.
EDIT:
this extract from include/kernel-defaults.mk is the code generating the checksum:
define Kernel/Configure/Default
$(LINUX_CONF_CMD) > $(LINUX_DIR)/.config.target
# copy CONFIG_KERNEL_* settings over to .config.target
awk '/^(#[[:space:]]+)?CONFIG_KERNEL/{sub("CONFIG_KERNEL_","CONFIG_");print}' $(TOPDIR)/.config >> $(LINUX_DIR)/.config.target
echo "# CONFIG_KALLSYMS_EXTRA_PASS is not set" >> $(LINUX_DIR)/.config.target
echo "# CONFIG_KALLSYMS_ALL is not set" >> $(LINUX_DIR)/.config.target
echo "# CONFIG_KPROBES is not set" >> $(LINUX_DIR)/.config.target
$(SCRIPT_DIR)/metadata.pl kconfig $(TMP_DIR)/.packageinfo $(TOPDIR)/.config > $(LINUX_DIR)/.config.override
$(SCRIPT_DIR)/kconfig.pl 'm+' '+' $(LINUX_DIR)/.config.target /dev/null $(LINUX_DIR)/.config.override > $(LINUX_DIR)/.config
$(call Kernel/SetInitramfs)
rm -rf $(KERNEL_BUILD_DIR)/modules
[ -d $(LINUX_DIR)/user_headers ] || $(MAKE) $(KERNEL_MAKEOPTS) INSTALL_HDR_PATH=$(LINUX_DIR)/user_headers headers_install
$(SH_FUNC) grep '=[ym]' $(LINUX_DIR)/.config | LC_ALL=C sort | md5s > $(LINUX_DIR)/.vermagic
endef
(Last edited by hnyman on 16 Oct 2012, 22:01)