Jerry - To answer your question, yes, it can be both.
Z-27= Super Sport Package: incl special engine, dual exhaust, springs, tires, clutch, radiator, MC1 transmission (in 69), axle and u-joints, starting motor, hood badging, and stripes. Also special ball joints, front springs, shocks, and engine mounts on 396. Wasn't referred to as Z27 until 1969 model. Base Z27 engine was L48, 396 optional.
X55 simply means an SS350 or L48.
As for your RPO N40/N44, again, the answer is both.
The link provided by Daniel explains it all in detail. All of the RPO's are listed on the left hand column. If there is an RPO you are unsure of, simply state what it is and we can help you determine if it came with your car or not. There are usually tall tale signs somewhere in the car that gives this information away.
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