Hi ET,
Been a while since I played with DF pipes, and I've given most of mine away so cannot compare, but my best guess is:

1) Not sure about this one. The front (header) end looks like the original JMP, made by MACS I think. There used to be aversions with 3 outlets made for an OS65 DF, but don't have the dimn's anymore.
How about asking Tom Cook - he may be able to confirm, and has been around the jet scene longer than many of us !

2) It looks very much like the pipe that Philip Avonds used to supply for OS91 with his very early F-15's, and later supllied it with the 2 stingers shortened a bit 'cos otherwise they vibrated off if the fan unit was badly balanced.
Think that MACS made them for him? Maybe shoot him an email with pic and Dimensioins and he should be abler to tell you for sure. If it is, then I can tell you it worked VERY nicely with OS91 and Ramtec fan. Would turn about 22,40 on a good day !

3) Looks really like an OPS pipe, maybe for their OPS80 (nice motor) or the 60 size boat motor. Is it baffled, or not?

Happy Landings,
Mike C