I think your IA may be incorrect in his assumption that the 150 was never certified without a spinner. In the TCDS for the 150 and 152 (same TCDS for both) a spinner isn't mentioned on any of the 150 models. A spinner isn't required until the 152 model. (page 10 of the TCDS).
Ask him/her to show you where it shows that the spinner is required per the TCDS.
Attachments
3a19 Type Cert.pdf (71.51 KB, 10 downloads)
TCDS for Cessna 150 and 152

Ron Stewart
N5282B
KSFZ


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