What's the difference ? Are they interchangeable ?  I'm in the process of acquiring 2 - 12v M-85 electronic ballasts . I was wondering if I could run my 70w MH PAR-38 flood using one of these ballasts . I've noticed that most of the electronic M-85 ballasts I've seen are rated for both the M-85 or M-98 lamps . These ballasts have no specs written on them . The original lamps included with the ballasts are Venture MH-DE 70W/UVS/4K double ended tubular lamps .

I looked into this and it looks like M85 is the code used for the G12 based and double ended versions of the 70w MH lamp, while M98 is typically the screw based version of the 70w lamp.

The lamp currents and voltages are more or less the same with both having a nominal 0.9A current and nominal 95V lamp voltage. If you're willing to test them, I'd say try them out. Do you have any pictures of the ballast? Perhaps the manufacturer might be able to give you more information. For electronic HID ballasts, sometimes they have an "approved" list of lamps that they can provide you.

Interesting. A while back, I bought a daylight double ended 70W Osram MH lamp with the intention of using it on one of my M98 ballasts. Someone here (I don't remember whom) advised me against it because the electrical specs are close, but not close enough (and I don't remember exactly what).

I moved off to other projects since then, but every now and then I come across that lamp in my lighting things and think of getting it going. I have to find a suitable fixture because operating non-protected MH lamps is out of the question in my experiments/creations. One of these days, I might get around to finding a suitable halogen fixture to convert and give it a try on a M98 ballast.

Oh, and I've always thought G12 lamps of 70W were all M98... (I don't have any, though).

Hmm, interesting my conclusion was the difference in lamp current and voltage was more or less within the variance I've found between manufacturers. I suppose the different ANSI codes are for professional applications where more critical current regulation is desired to avoid variances in colour. 

Philips CDM70/TD/830 C139 94v @ 0.95A 
Philips CDM MHC70/U/M/3K ELITE C98 95v @ 0.91A
Philips MasterColor CDM-T 70W/942 G12 T6 CL C139 88v @ 0.98A
Philips MasterColor CDM70/PAR38/FL/3K ALTO ELITE C98 95v @ 0.92A

Venture MP 70W/U/PS/EM/950 M98 85v @ 0.88A
Venture HIT 70W/G12/UVS/4K M98/M85 95v @ 0.90A
Venture MH-DE 70W/UVS/4K M98/M85 95v @ 0.90A

Well, I suppose if exact colour rendition is required (or within very tight tolerances) then it would make sense to have a dedicated M98 or M85 ballast. However, in most applications where exact specifications are not absolutely required (such as a single lamp in home use), then these are within acceptable mains variations.

I think the lights were made by Fire Research Corp. They looked like this : https://www.lighting-gallery.net/gallery/displayimage.php?album=5198&pos=73&pid=177520 . As they are rather large , the 2 I most recently found at worked got taken apart and I salvaged the lamps, lamp holders and ballasts . The ballasts themselves are nothing more than sealed black boxes . I'll have to look closer Monday for any numbers . Plus , I think those lights actually had labels on them.

This is the electronic ballast I found on eBay that lists both M-98 and M-85  :  https://www.ebay.com/itm/293152560830

 If worse comes to worse , I could just use the lamp and ballast in conjunction with a 500w Halogen flood with the ballast remote mounted . A 70w MH 12v flood would be nice to have .

I guess the ballasts might be manufacturer specific so maybe check the lights if you happen to find one with a label.

I have a few 70w MH ballasts, the magnetic ones (Venture V90D5832) are listed as M98 only, the two electronic ones I have (Advance IMH-70-G-BLS) list M98, M139, and M143 on the label.

Worst case you can probably find a compact 70w MH flood designed for G12 style lamps too. Yeah a 12v MH flood is pretty cool to have, I've considered building one with a car headlight kit and retrofitting in into a standard HID flood housing.