Looks like you've got the press-fit cartridge bearing bottom bracket. I hope the you've also got the spindle. Anyway if this is the case, your crankset choices are limited to square-taper ones. Maybe look for a used or NOS Raceface Turbine LP crank or check out Middleburn--I believe that still make a square taper model or two.
I call BS. I had a Merlin Mountain bike frame that I purchased new in 1989 or 1990 (i forget) but the serial number was 957. Same pressed in BB bearings with the spindle that had a collar on each end. The frame was made for a short rigid fork with a one inch steer tube. My vote would be a single speed build or 1x9 for trails or city coffee crusing.
I just found and bought an old Merlin MTB complete bike. It is said to be from 1989 the serial #1083
It's equipped with the following spec:
Grafton cranks, Shim. oval chain rings
Titanium stem, post, spokes, bolts and handlebar
Bulls eye hubs
Mavic ceramic rims
Flite Ti saddle
XT tumbies and derailleurs
Hollow pin America chain
Dia compe brakes and levers and a strange "knee/canti/ Ubrake" in the rear
The strange thing is, it's completely unused! Even the Ritchy Megabite Zmax Championchip Series tires still have the grainy white stuff all over them. The brake and gear cables are still not connected.
Since I have owned and loved a Merlin MTB since 1994 I just have to bye this one.
The dilemma is; Should I start use this one? Now I use my old one as a commuter and I love it! But my wife want one as well...
What do you think? Is it a crime to take its virginity?
Yes it got the press fit BB, very slender shell and a straight forward souloution. In the box that followed the bike were also an extra CroMo axle ( Titanium fitted), a 8speed Tioga Titanium cassette, an envelobe with two sets of original decals, Ringle Ti scewers, Ringle bottle holder.
The Chain is cromed on all surfaces and came in a cylindrical red tin box.
The rear brake isn't a roller cam, it got a "knee" pointing down and when the wire pull in the knee pivot it presses outward.