MCDU data lock comes at 700 ft, a slightly different thing.
Originally Posted by FCOM DSC-22_30-80-30-10 Land Mode
LAND mode automatically engages when the LOC and G/S modes are engaged, and the aircraft is below 400 ft RA. The FMA displays “LAND”, indicating that LOC and G/S are locked. No action on the FCU will disengage LAND mode. FLARE and ROLL OUT modes will successively engage.
LAND mode disengages:
‐ Upon engagement of the go-around mode
‐ If the flight crew presses the APPR pb, when the aircraft has been on ground for at least 10 s with the autopilot disconnected
‐ When both AP/FDs are disengaged.
Note: When LAND is not displayed on the FMA, at/or slightly below 400 ft, the landing capability degrades to CAT 1 and the triple click is generated. No autoland is authorized with CAT 1 displayed on the FMA.
Originally Posted by FCOM PRO-NOR-SRP-01-70 Data Lock
When the aircraft reaches 700 ft RA with APPR mode (LOC and G/S) armed or engaged, the ILS FREQ and course are frozen in the receiver.
This function (ILS tune inhibit) is available, when at least one AP/FD is engaged. Any attempt to change the ILS frequency or CRS, via the MCDU or RMP, does not affect the receiver.
If the speed is managed, the system does not accept any modifications the pilot may enter on the PERF APPR page (surface wind, selected landing configuration, or VAPP) for speed guidance purposes below this altitude.
When the aircraft reaches 400 ft RA, LAND mode engages. The flight crew can only disengage this mode by engaging the GO AROUND mode.
B737NG_Pilot You seem to have taken a very fine comb to extract information from the FCOM, sir. It's admirable, godspeed.
Welcome to the hive.