Before or after
If you read the USCG document that I just attached, I think you'll find a discussion as to how the HINs changed after 1984...prior to that (1972-1984), there was a different set up for the HINs & I believe it is discussed in that document.
If you know the model year of the vessel in question, you'll know which convention to follow.
From here on is additional text added by editing...after stating the above, I quickly did a read of the article & while it mentions how the last 4 characters differ in the two conventions of constructing/encoding/decoding HINs, it doesn't provide much of a description of how to decode the last 4 characters of HINs generated prior to August 1, 1984, if the use of letters is employed.
So, I've since found a description of the 2 ways to construct/encode/decode a HIN prior to 8/1/1984, and I'll use your HIN as the example:
ERY 29381M75A. The first 3 characters are the code for the manufacturer...this we already knew. The next 5 characters, or positions 4 through 8, are the serial number for the boat in question...I guess Ericson chose to use the first 2 of these positions to inform one as to the putative loa of the model in question...so that yours is a 29-footer & then comes the 381 serial number for 29 footers made by Ericson. Now we're at the last 4 characters. From sometime in 1972 to 8/1/84, these could be represented in 2 ways...either the model year or the certification date. If the manufacturer chose to provide the certification date, the last 4 characters would all be numbers, with the first 2 representing the month of the year (starting w/ 01 corresponding to January) & the last 2 corresponding the decade and year (i.e., 75 would be 1975). Now, if the manufacturer chose to use the MODEL YEAR designation, they would use the letter "M" in the 9th position to indicate this. The next 2 numbers would be the decade and year of the model in question...in your case 1975...and the last character would be a letter corresponding to the month it was manufactured or certified (I'm unclear about this point) with A=August, B=September,... ...,L=July. This letter designation for the month in question changed as of 8/1/84 so that A=January, etc.
This site explains both extremely well & a more concisely than I did
:
http://www.calyachts.org/MaintanceShop/HIN.html
I hope this helps.
Regards,
Ed