NAB vs. IEC
The NAB eq curve, used in North America, employs time constants of 3183 usec (50 Hz) for the bass, & 50 usec (3183 Hz) for the treble, at speeds of 7.5 & 15 ips, in playback mode. This spec originated in the early 1950's, when tapes had less hf capability, & bass boost in recording was employed at 50 Hz to avoid lf rumble during playback.
The IEC curve came a little later & is, IMHO, a curve which better matches & better utilizes the benefits of improved tapes. It uses an infinite time constant in the bass end (0 Hz), & a 35 usec (4547 Hz) time constant in the treble region at 15 ips speed. At 7.5 ips, bass is the same, but the treble time constant is 70 usec (2274 Hz). Again, IEC eq provides less treble boost in playback (at 15 ips speed), realizing a slight improvement in s/n ratio over NAB eq. The difference is about 1.0 to 1.5 dB. At 7.5 ips speed, NAB provides less treble boost & a little better snr, 1.0 to 1.5 dB.
A tape recorded at 15 ips w/ NAB eq & played back w/ IEC eq, will likely sound a little deficient in the treble, as well as in the deep bass. But the amount is not that great, like I said a dB or so. A tape recorded in IEC, then played back in NAB will have slightly boosted treble & bass. The freq response is not flat this way, but there will always be some listeners who prefer boosted sound at the bass & treble regions.
If the recording speed was 7.5 ips, then IEC provides more playback treble boost than NAB. So the treble response will vary in the opposite direction as above, but still a small difference. Bass remains the same as above.
If "flat" is the objective, then I recommend playing back w/ the same eq curve used during recording. Still, the difference is pretty slim, & if the right curve is not available, a parametric or graphic outboard eq can restore flat response. Use the break frequencies given above & all should be well. Again, if an outboard eq is not available either, no big deal. The deviation from flat response is too scant to be concerned about. I hope I've helped. BR.