Somehow, I think that it's much more likely that someone made a mistake filling out the airworthiness certificate than they did filing and approving the whole type certificate. Look it up on the FAA's online Regulatory and Guidance Library for yourself. If you search for "UC-3B" nothing at all comes up - literally "No Documents Found" - but if you search for "UC-1" it takes you to TC no. A6EA - on which the only model listed is the "UC-1".
http://www.airweb.faa.gov/Regulator...1d9adbbe6685c38525673e0059a495/$FILE/a6ea.pdf
Also, if you do a FAA registration search by Make/Model for Mfg. = STOL and model = "UC-3B" once again the results are "no records found" but if you search for "STOL" and "UC-1" then 13 records come up.
Maybe you ought to have a discussion with your local FSDO and/or the regional ACO. That CoA may not be valid.
And BTW, they made 23 of them - serials 001 through 024, skipping no. 013. Well, actually, the first several had serials that reflected their former identities as models RC-3. There was UC1-R158, which indicates that it was converted from RC-3 s/n 158. There was UC2-R1041, indicating that it was converted from RC-3 s/n 1041. There was also UC3-R362 (from RC-3 s/n 362) and N9501U was registered as s/n UC006. (The 4th and 5th ones built were lost so long ago that I couldn't find any old registration records for them.)
The rest are in the FAA database as just serial numbers 008 through 024. (Actually s/n 024 is no longer on the FAA registry. It was N77GT but it went to Switzerland to the guy who also owns HB-LSK, s/n 018, presumably to use as a parts airplane.)
The 12th one built, that is serial number 012 according to the FAA database, is
N123BR. It is registerd to the Rojean Corp of Santa Ana, CA and it is listed there as a 1972 model UC-1 (s/n 012). See for yourself:
http://registry.faa.gov/aircraftinquiry/NNum_Results.aspx?NNumbertxt=123BR