So, from your sentence, "the lender has come back....", meaning you have not seen or have possession of the IBTS letter, and your info is coming from a 3rd party (the lender).
Now that we have some additional information, I would:
Ask for the IBTS letter for my work file, provide a copy in the report.
Attach a copy/photo of the data plate in the report. (using this as support calling it a manufactured)
Subject to engineering certification. (to confirm manufactured by providing HUD tag info on certification) <---they will have access to the frame while inspecting
Pass the verification 'ball' to someone else. You have EXHAUSTED your scope, your expertise, your general course of business in gathering data. Someone (appraiser) messed up in the past and now the lender wants YOU to fix it.
And, I may not be right, but this is how I would handle it to both protect myself and the client. I wouldn't call a manufactured home modular just because a data plate is missing some numbers. NOPE.
GOOD LUCK