Some low speed recordings do not have a standard speed, therefore it can only played back on the VCR or at least the brand of VCR that made that low speed recording. I've had tapes like that in NTSC and was able to play them back in a multi-standard multi-scan VCR, but don't play on a single scan standard VCR, My multi-scan VCR don't even register what speed is like SP, LP, EP, the indicator is completely off with those oddball tapes but playback is fine.
What I mean by multi-scan VCR is the scan speed of 525 and 625 native recordings, The capstan and drum motors have variable speeds on those machines, Unlike other machines that are set to either 525 (60 fields/s) or 625 (50 fields/s).
Not all multi-standard VCR's are multi-scan, Some use color standard conversion schemes such as PAL60, NTSC 4.43 or convert to another color format entirely, but cannot output the native frame rate. So if you can get one of those VCRs you should try it.