Maybe January 1974 build date, serial number 773?
From Jim Hubbard's old site:
HULL IDENTIFICATION NUMBER/SERIAL NUMBER:
There are two numbers associated with each boat. One is its legal "serial number", called properly the Hull Identification Number. It is the number on the title of the boat. The HIN is located on the transom, starboard, high up at the corner.
The other is the sail number (SN for short). There are two places to find a SN on the hull that I know of. If anybody knows of another, by all means speak up. On the very early boats, below about SN 450 or so, the SN is stamped into the metal fitting in the companion way cover through which the companion way board tang fits. Above abut SN 450, the SN is stamped into the metal plate that covers the opening where the backstay tang comes up through the transom.
For the later boats, after about SN 1700, there is a simple relationship between the SN and HIN. Before that it is not always obvious.
The general rules for decoding HINs are given on the web at
http://boatsafe.com/nauticalknowhow/hin.htm. Supposedly these apply to all boats built after Nov. 1, 1972 but most of the 1972 SJ21 numbers that I've accumulated don't seem to fit.
The approximate rules that I have deduced over the years are these:
a) Early, west coast built boats have HINs in the form CLK215330474. This is the actual HIN for SN 0701. (Leading zero stuck in for clarity.) NOTE that the SN does NOT appear in the HIN. The trailing 4 digits are the month and year of construction, April 1974. "CLK" obviously is Clark and 21 is obvious also. The rest is apparently a factory serial number.
b) Beginning somewhere in 12974-75, the numbering pattern shifted. i. East coast built boats continued with the CLK21 + serial number + month + date. Thus my boat, SN 1294, has HIN CLK217070575 ii. West coast built boats have either CLKC or CLKE + SN + month + year For example SN 1150 is HIN CLKC11500675 and SN 1217 is CLKE12170375. iii. But there are exceptions. SN 1324 for example is CLKE12220375 which should have been SN 1222.
c) By 1979, the HINs had shifted again. All of them read CLKC or CLKE + SN + letter M + yr + month-letter, where month letter is A=August, B=Sept., C=Oct. D=Nov., E=Dec., F=Jan., G=Feb., H=Mar. I=Apr. J=May, K=June, L=July. Examples: SN 2037 is HIN CLKC2037M78D, SN 2059 is HIN CLKE2059M78F. After 1978 they all seem to use CLKE.
d) The legal requirements for HINs changed on August 1, 1984 but I do not have any SJ21 HINs after that date in my file.