In fairness, there's a good bit of circumstantial TOS justification for Klingons with cloaking devices. The Romulans were using three D7s in the episode The Enterprise Incident, very soon after they'd just lost the subwarp border-raider to the Enterprise. The Romulans modernized quickly, clearly getting ships from the Klingons, which worked just fine with the Cloaking Device. And they'd have to trade something to the Klingons to get those ships; while AFAIK it's not stated directly in the episode the implication is certainly possible.
That's actually something I found extraordinarily stupid in "Balance of Terror"...saying the Romulan
Bird of Prey just had impulse propulsion, which everyone translates as being incapable of warp speed...if the ship can't go faster than light, there was no way one ship would be able to hit all those listening posts, much less made the trip from Romulan space in the amount of time it would have had (it would have taken decades, if not centuries to make it between stars...space is BIG after all)...In this instance would agree with the Starfleet Battles interpretation of this, and that they did have warp capability, it just wasn't of a type at which you could fight...
I could easily see the Klingons trading ships and warp combat technology for cloaking technology...and maybe even the plasma torpedo technology (like they did in the FASA RPG)...
BTW, speaking of the D7 and looking predatory, take a look at the image in the middle of this page (taken from some book in the FASA RPG if I'm not mistaken):
http://www.ststcsolda.space/klingons/D-7/D-7.htmlAbsolutely love that image...although the stats of the ship in the game never seemed to live up to the vessel's hype to me...should have had at least a forward facing torpedo tube (if not a rear facing one as well), and two more disrupters (which I think I would have as rear facing)...would have loved that vessel...
Ruger