Unfortunately this question without having more info is basically impossible to answer. I'd recommend do fitting and having new shafts installed (I know really not the answer your looking, and is of zero help ).

However, I've played the Project X 5.5 and was one of my favorite steel shafts due to the lower flight, spin, and weight, given the shafts profile which I prefer. If your looking for a lighter shaft than yes the Nippon 1050 (105g weight) may be a starting point demo for a lighter weighted shaft. Granted when I played the 1050 it was with a set of Scratch Tour Department EZ-1/AR-1 combo set which had similar playing characteristics to the Adams A4 TOUR heads which had PX 5.5 shafts. I found the Nippon 1050 to really go high, and spin a lot to point where the ball would basically stall out in the air and drop out the sky. Again not an apples to apples comparison at all, but some close (head wise).

I did pick up a used set a AP2 712 with Aerotech Steelfiber i95 stiff shafts. I planned to pull them immediately and put PX 5.5 in them but decided to give them a go before pulling them. Personally, for "me", I'm glad I tried them because I loved them! I thought I'd hate them but they really do have great feel, predictable like the PX 5.5, but considerably lighter, but play like a steel shaft but with a high end driver graphite shaft feel/smoothness. I liked them so much I ordered my AP2 716 with them straight from Titleist. I'm not a real longer hitter, but I did gain about a club which was great, but my dispersion was just as good as it was PX, and other main steel shafts out.

Again, this is just what worked for me. If your looking for a lighter alternative but has low/mid launch, low/midish spin like a PX might be worth demoing. There are now graphite style iron shafts that are in the range of weight your looking at that are not whippy noodles like they used to be.

Also there is Project X PXI as an alternative too. Even at 5.5 flex they are close to the 105g you mentioned, 108g actually.

But if you do decide to try a Steelfiber, UST Recoil, Mitsubishi shafts u may find that lighter weight your after (although I've never tried any another but have read some good things.)

I'm 35 years old, and started when I was like 6 years old. So I'm somewhat young but have been around golf long enough, and the thought of a graphite iron shaft was never ever on my mind, ever, graphite Iron shafts were "made for" crappy whippy shafts, but they've really progressed.

So I'd say just go test tons of shafts, of all weights, and materials with an open mind, with a good reputable fitter on a good launch monitor. Find what gives you great numbers and what feels right in the weight range your after.

Best of luck.
Ryan

I just got out of my KBS Tour 90 Stiffs because they were too light/flexible for me but they have awesome high launch, feel, feedback for the 712 heads.
Cheers