Ironic, cause I just changed my signature last night.... come to find out my US90 is one of the first couple thousand made! Definitely a 1970... not a 71. According to that www.atcheaven.com site, they started first production with US90-100122. Also, mine has the plastic thumb throttle and the "Off-On" switch instead of the "Off-On-Off" switch. Definitely one of the first trikes built!

Originally Posted by
http://www.atcheaven.com/Model_ids.htm
ATC90 (US90) 1970-71 (K0)
The first ever ATC90, named the US90 as identified by the engine number prefix of US90. Produced from late 1969 through 1971. Distinguishing features: No headlight or tail-light, Honda name on tank in badge form instead of decal, plastic thumb throttle (prone to breakage and replaced under warranty with alloy unit)
7 digit engine number US90E-100122~
Wonder if Honda would still honor the warranty? hahahaha
From that same site, the 73 model (K1) started with Engine number US90E-2000001~
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