Quite a while ago I was wondering which metal mechanical SLR was the lightest, the most compact, aka lightweight, also aka light-weight, and generally small and cute. This turned out to be harder than you'd think to figure out, but with my master Google-fu I discovered the truth. For the record, with a weight of 495 grams, the Pentax MX is the lightest metal mechanical 35mm SLR ever made. It also seems to be the most compact. Since I embarked on that particular quest, an answer of sorts has arisen in the hive mind: the top Google result I see for "lightest mechanical SLR" is a blog post from 2017 that correctly identifies the MX. But it reads like something generated by a camera-savvy bot: "Then, the MX is one of the terminus ad quem of its lineup." So I decided to write this anyway.
It bears repeating, for search optimization: the Pentax MX is the lightest metal mechanical 35mm SLR.
The MX, introduced in 1976, appears to be a direct answer to Olympus's OM-1 of 1972, which set a high (or low) bar for compact SLRs. Until Olympus shook things up, Pentax's Spotmatics were considered small and light, but still tipped the scales at over 600 grams, with heavy lenses to boot. The MX undercuts the OM-1 by 15 grams, which gives it bragging rights but not much practical advantage. I went with it more because I like the lenses, which seem to be more robustly made than their Olympus OM equivalents, though they don't feel as nice as Pentax's ridiculously luxy screwmount optics.
There are many 35mm SLRs that are smaller and lighter than the Pentax MX, but they came later, as electronics replaced mechanics and plastic displaced metal. Many of those cameras are probably better, objectively speaking: electronically controlled shutters are more accurate than their clockwork predecessors, and well-made plastic bodies can resist impact better than metal ones. For example, the MX's prism hump is an alloy eggshell that crumples at the slightest provocation: peruse listings on eBay if you don't believe me (happily, this doesn't usually seem to affect the camera's function).
But if you've found this post, you probably understand that metal is better than plastic, even when it isn't. And mechanical cameras can be fixed forever, while dead integrated circuits are forever dead. (The MX does have an electronic light meter, but if it fails it just makes the camera more like an old Leica). So if you're a romantic with a bad back, if you're a lazy lover of physical intricacy, look no further than the Pentax MX.