I've got a Lee Custom K&B .61, which I've owned nearly 45 years. Great running engine. First used it to power a model called a Taurus, which I initially flew with a second-hand Fox .59 (two glow plugs in that engine). Subsequently, I installed it in a Phoenix 5 (another pattern plane, but this one with a fiberglass fuselage, the first of many, and my first experience with retracts - Rhom Air).
With the advent of more powerful Schnuerle-ports, old fashioned loop-scavanged engines like the Fox and K&B lost popularity - except for Clarence Lee's really good running K&B .61. Anyway, I later used it to powered my Andrews Aeromaster where it lived maybe another 5-6 years before I damaged the crank in a crash. This was 1983 and happened due to switch failure. Interestingly, said switch failure directly led to ProModeler because in my frustration I soldered a second lead to my battery pack so I could run two switches - after all, the odds of both switches failing on the same flight are astronomical.
Then a mentor with a Royal Cessna 337 (push-pull), which years ago I had built for him (architect, now deceased but with more money than God), asked me to do the same for him. Then I made about 10 more packs for the rest of his models - and thus, my first experience buying something (cells and leads) from other companies for resale happened (bear in mind, at the time I'm just out of high school). Soon another scale modeler wanted me to do one for him (this was when scale models began as a box of sticks, and consumed months of building - the point being this was when the risk of a crash meant more than today when another ARF can be sourced, assembled, and flown inside a week so the prospect of increased reliability for the price of an inexpensive switch made great sense).
Fast forward to today and we still offer dual-lead batteries plus really good quality switches ($10). Speaking of switches, now they're heavy duty (at 20A they're rated for four times the current and are more reliable). Moreover, in addition to redundancy, these days two switches means the receiver can draw twice as much current (10A total) as through a single lead (rated at 5A) so this is another important - added reason - for using two switches.
Anyway, before long folks from a club across town caught wind of what I was doing and just like that I had a small sideline business. One, which like today, grew based solely on word of mouth! Note, this first product was a dual-lead 500mAH 4-cell NiCd battery pack and today (fast approaching 40 years in the hobby business), I continue offering dual lead packs (as do plenty of others). Note; while I make the claim of being first (although being a youngster, it never occurred to me to try and patent it, added to which, today I also know it would have been impossible as a practical matter to defend said patent even had I thought of trying to obtain one) finding other suppliers isn't difficult, or rare.
Meanwhile, present day packs in our ProModeler line-up range in capacity from 1000-5000mAh are still popular because using two switches continues as my single best suggestion for protecting your model (switches remain the most common point of failure in my opinion because, these days, receivers and servos use surface mount PCBs, which are uber reliable compared to back-in-the-day practice, when discrete components hand-soldered were how things were done).
But back to Mr. Lee, I returned the crash damaged engine for him to straighten the crank and purchased another carb (it had sheared off). Upon its return - looking and running as good as new - next, it powered a Great Planes Sportster before ultimately finding a home on a Pica Waco (a scale .61 size biplane) where it lives to this day! Lots of good memories.