I compared the header keywords (not values)  between chip1 and chip3 of each
of the 187 associations that I have here on disk.
Here the results:

160 cases:

  chip 1 and chip3 differ because there are 4 COMMENT lines in chip 3 not available in chip 1

23 cases:

the diff between chip 1 (<) and chip 3 (>) is:

254,259d253
< WCSDIM
< LTM1_1
< LTM2_2
< WAT0_001
< WAT1_001
< WAT2_001
308a303,306
> COMMENT
> COMMENT
> COMMENT
> COMMENT


The remaining 4 cases:

diff.u2bi0101b
305a306,309
> COMMENT
> COMMENT
> COMMENT
> COMMENT
310,322d313
< IMATYB
< IMATYV
< IRATYV
< SGSEP
< MAXMERR
< FR_LOWF
< FR_STAR
< FR_GAL
< F_LOWF
< F_STAR
< F_GAL
< F_TOTAL
< IMATYE

diff.u27q0108b
253,260d252
< WCSDIM
< LTV1
< LTV2
< LTM1_1
< LTM2_2
< WAT0_001
< WAT1_001
< WAT2_001
309a302,305
> COMMENT
> COMMENT
> COMMENT
> COMMENT


more diff.u2m7040mb
252,257d251
< WCSDIM
< LTM1_1
< LTM2_2
< WAT0_001
< WAT1_001
< WAT2_001
262,328d255
< CVOEQUI
< CVOSUNIT
< CVOSFILL
< CVOSPAE
< CVOSSAMP
< CVONYQR
< CVOSRES
< CVOWUNIT
< CVOWMIN
< CVOWMAX
< CVOWSPAN
< CVOWSAMP
< CVOWSPAE
< CVOWSAME
< CVOTUNIT
< CVOTFORM
< TEXPTIME
< CVOTSPAN
< CVOTPAE
< CVOTERR
< CVOTFERR
< CVOTSAME
< CVOTSPAE
< CVOTMIN
< CVOTMAX
< CVOARCH
< CVOARCID
< CVONVER
< CVOFUNIT
< SKYVAL
< MEXPTIME
< CVOGAIN
< OGAIN
< CVOCROT
< PIXSCALE
< CVOWERR
< CVOSERR
< CVOPSFW
< ZEROSTS
< ZEROSTE
< ZEROABS
< ZEROABE
< CVOMAG0
< COMMENT
< COMMENT
< COMMENT
< COMMENT
< IMSTAV
< IRAISV
< IMSTAB
< IMSTAE
< IMATYB
< IMATYV
< IRATYV
< SGSEP
< MAXMERR
< FR_LOWF
< FR_STAR
< FR_GAL
< F_LOWF
< F_STAR
< F_GAL
< F_TOTAL
< IMATYE
< SEXCV
< SEXCB
< SEXCE


diff.u39u010eb
255,261d254
< WCSDIM
< LTV2
< LTM1_1
< LTM2_2
< WAT0_001
< WAT1_001
< WAT2_001
310a304,307
> COMMENT
> COMMENT
> COMMENT
> COMMENT