Can anyone tell me the RG and Diff of the old Brunswick Mark X?
What ball available today would match up with it the best for reaction?
Anyone have one for sale?

Thank You

Yep about any plastic ball, would replace the Mark X it's plastic ball.

Unless you mean the "Maxx X":
Reactive Solid with a Symmetrical core
Minimum RG    2.480
Maximum RG    2.522
Total Differental    0.042

I remember the Mark X being a ball that Carmen Salvino popularized. It was great if you used it on a shot 10-board out to the gutter. Anything inside of it the ball was a putz. (This also reminds me of the Columbia Orange Dot, its first foray with urethane: great if you were playing 10-board out to the gutter.) Also, the Mark X cover was very brittle and cracked; that was the second biggest complaint about it.

Besides, the LT-48 was such a game changer, who needed a Mark X?

The main reason they cracked were due to the insecurity of the weight blocks in the mold...

The cover was brittle as most soft polyesters were but most if not all of the cracking was created by the instability created by the inconsistency of the weight block placements

Look at the winners from the 81-82 season on tour....out of 16 it was over half that won using that ball including Earl and Cook won the ToC

Strikeking, some of us are old enough to remember the original stuff when it was introduced. If there was a urethane or reactive incarnation of the ball, I didn't recall there being one. It sounds like the Mark X you're describing is akin what's been done with the Blue Hammer and LT-48. I'll just leave it at that.

Well I'd say it's to the point where your best bet is to email Brunswick and ask them.

It's unlikely though there will be any info on RG or Diff. for a ball that old.

(A poly Ball from back then, the RG will be high and the diff. practically nothing.)

As for the weight block, you can have a 2 piece "Pancake" . There was a dual density one I remember. One half was heavier  than the other lighter side .

I'm not saying that's what it is and hey maybe that isn't the correct core picture for that ball. Just that it's possible.

Interest note: according to the Mark X ad, it was "Dynamically Balanced to Eliminate wobble"

Wobble was what they called Flare back then (and was thought to be a bad thing), so it was designed not to Flare.