Everything that was said above pretty much covers the rifle, butlet me add a little more...
There are many variations of the 618 and I still haven't found them all! (I now have 4 1/2, 618's).
From what I have seen, the internals changed very little as the Model evolved. Basic single stage trigger, rolled tin rear mainspring guide, and crudelly finished, leather sealed, piston's are inside. Recievers come in two flavors, scope grooved and non-grooved. The barrels are what make some models of these rifles shine!
I'm probablly going out on a limb here, but it appears to me that there are two basic models, the "Standard" 618 with an un-grooved reciever, more open grained stock wood and on some models thhere is the old style CZ chamber ring (for holding "bulk" round lead shot. Popular in Europe as our Steel BB's are here) and what I loosley term as the "LUX" version. This one begins with a more denslly grained European hardwood stock (usually have nice highlights too), the reciever is grooved for the elusive and diminutitive CZ 4X scope. The BBL is smoothly rifled all the way, breech to muzzle (obviouslly intended for pellet-only, use?).
Internals on all models are the same, as are the mainsprings. ALL could benifit from some smoothing of the stock components, and if one had acess to, and the knowlege ofhow-to, new guides could really improve things. Trigger mechanicals operate under some pretty small safety tolerances, and good judgement dictates very minor work there. The rifles can easilly be rendered un-cockble or un-safe, from even minor metal removal here, OK? Far as common velocitys go, a real good condition 618 can shoot between 360 to 395fps, tho most go 320-360fps, in .177. One of my "pet's", a reciever sighted, well tuned and smoothed, Raven-18 Model, can group it's low velocity pellets into cloverleafs seated at 20m benchrest distance (I doubt this rifle cracks 340fps, but is it ever accurate!). More common is 1 inch at that distance with an average 618. Just today, I was ripping up on a 20 yard soda can, offhand, with a "Standard" model 618 and Crosman SuperMatches. These 618'sare real fun guns, and make ideal plinkers.!
Apparentlly the 618 was very prolific in Canada, and can be found in good condition, at local Gun Shows for as low as $12-$15 CDN! So don't bid too high on this one. Here in the US, all these earlier CZ's were less available than elswhere, but importers such as "Precise" brought many here too.My old ZVP pistol was brought here in the 60's, so you can see the company has made many tens of thousands of various models, spread worldwide!
The basic charge for the 618, is as a boys rifle or beginners gun, and many show the sufferages of their youthfull owners abuses. Bent bbls are common, as are damaged pistons from errant firings with the barrel not at battery. Nevertheless, many of these sturdy little CZ's are still in service all over!
All European manufacturers built some sort of Beginner grade rifle, Diana being the most seen versions. Even here in the USA, Crosman made the wonderfull 760 pneumatic, with the same intentions in mind.
It's strange how much all these beginner guns are alike! When one gets to collecting a number of various make beginner rifles, the common OAL size pattern and LOP are the same, as are sight radui, trigger pulls and for the springers, basic velocitys. Somewhere there must be a a chart... LOL!
Lewis Reinhold of Beemans Australia carries all the seals for the small CZ's, so re-build parts can be ordered still.