I have some NEW LAMB tubes give me a PM me if interested.
Why, are they metric? Because it is made in CHINA and these are not aircraft tires. They were made for other utilitarian uses (carts, wheel barrel).
380-150 was never used with andy normally certified plane or Cessna retract or fixed gear, even for the nose gear. Cessna used different mains on the RG's (172RG, 182RG...) than the fixed gear version of that plane.... but they are a version of 6.00-6's.
Most Cessna singles use 6.00-6 for mains and 5.00-5 for nose. The RG's used a 15x6.00-6 which as you say was to give more clearance in the wheel well. The 5.00-5 is the common nose gears on Cessna singles fixed or retract. This is standard of course for the RV's.
Personally I wsa going with the LAMB tires for mains on my RV-7 and changed my mind. I swapped my Sam James wheel pants out (before installing). I sold the tires. I have some spare tubes for LAMB tires. PM me if interested.