Following is a response from DHS staff member, Tom Pullum:
The following lines will do the merge for the 1990 files:
use "...NGHW21FL.DTA", clear
rename hwcaseid caseid
rename hwline bidx
merge 1:1 caseid bidx using "...NGKR21FL.DTA"
tab _merge
"hwcaseid" in the HW file is equivalent to "caseid", the woman's unique ID, in the IR and KR files. "hwline" is equivalent to "bidx". You can see the correspondence if you list hwcaseid and hwline for the first 50 cases in the HW file, and caseid and bidx for the first 50 cases in the KR file (for example, "list caseid bidx if _n<=50, table clean").