I've looked at a three different years available to me; 1967, 1969 and the 1977 Commuter. The only C150 POH that I can see, that gives 52 mph, is the 1967 Commuter. However, this is a figure for a maximum performance T/O at 50ft, thereafter the POH gives 72 mph for a maximum performance climb. These are all generic manuals and I did not have access to the NAA amendments.
All three models have the same engine and a 69 inch prop, I presume all are McCaulay. Each POH give the same wing loading, power loading and stall speed. There is a tiny difference given for the 1977 Commuter wing loading of 10.0 pounds/sq ft, all others being 10.2. The main anomalies seem to be recommended speed at 50 ft for a max. perf. T/O, although the other max. perf. climb speeds vary by the odd 1 mph.
For low powered single engine aeroplanes the figures seem to be the observations and prejudices of the test pilot. To be fair there is probably no other way to do it. No one appears to have looked back or forward for consistency. Aviation is fond of giving the illusion of precision but it usually isn't. There's an awful lot of shoulder shrugging.
Before EASA we used have the air test on every third annual (Star annual). The anomalies we found in manuals were common together with errors, which in my experience manufacturers were never keen to correct. Sometimes an NAA becomes aware of the mistakes and anomalies and requires amendments to be entered in the POH. I don't know if these discoveries were ever fed back to the manufacturers.