It sounds like a Scubapro Mk 7.
It is an old reg but none the less offers decent performance, although problems with the honking side of the reg are hard to resolve as parts are no longer made for it. The business end of the reg is a Mk 5 and annual service parts are still made for it.
The downsides of a Mk 7's are the lack of a suitable number of low pressure ports and the fact that it weighs a ton.
I'm not sure what second stage you have on it but it is mostly likley an Adjustable, Balanced Adjustable, Air 1, D300 or D350. All but the Air 1 are still supported by Scubapro and all of them would breathe as good as or better than just about any second stage you could buy now.
The octo used most often on them was the 080 "High Performance" unless the original owner spent big bucks and put on an octo that was the same as the primary. HP's are still supported and use R190 annual service kits.
If it were mine and had an Adjustable, Balanced Adjustable, D300 or D350 second stage, I'd just buy a new Mk 17 first stage and continue using the old second stage and octo.
If you have an "Adjustable" second stage, you can inexpensively upgrade it to the "Balanced Adjustable". The upgrade requires a new spring and balance chamber in addition to the normal G260/S600 annual service kit. The extra parts needed cost less than $5.00. If you upgrade it, it will give performance equal to the S600, just without the adjustable flow vane that no one adjusts anyway.
D300's and D350's can also be upgraded to D400's but it is not worth the cost asall three models already breathe exceptionally and equally well.